Why should one not address a venerable as friend? Here is a quote from MN 26:

One, standing up to greet me, received my robe &amp; bowl. Another spread out a seat. Another set out water for washing my feet. However, they addressed me by name and as 'friend.'
So I said to them,

'Don't address the Tathagata by name and as &quot;friend.&quot; The Tathagata, friends, is a worthy one, rightly self-awakened. Lend ear, friends: the Deathless has been attained. I will instruct you. I will teach you the Dhamma.


I can understand why the Tathagata might not be addressed by name; but why not as &quot;friend&quot;?
Perhaps similarly, from the Maha-parinibbana Sutta:

And, Ananda, whereas now the bhikkhus address one another as 'friend (āvuso)', let it not be so when I am gone. The senior bhikkhus, Ananda, may address the junior ones by their name, their family name, or as 'friend'; but the junior bhikkhus should address the senior ones as 'venerable sir (āyasmā)' or 'your reverence (bhante)'.

What's the need for, what's the benefit of, the &quot;reverent&quot; form of address? Or what's the harm, if any, in the &quot;friendly&quot; mode of address?
If you can, I'd appreciate answers:

From canonical sources (e.g. suttas or commentary)
From post-canonical sources (e.g. later, or modern, articles or dhamma talks)
From personal experience
and/or which apply to lay people (when addressing monks, or teachers or other venerables)


Edit:
I think that ruben202's answer is ample evidence that it is so, in the culture and in the suttas and other scriptures.
I'm not sure I understand why, though. For example:

A venerable is a friend -- or are they not?
Is the behaviour (or mode of address) mere ritual?
Is there said to be a benefit, some purpose, some effect: for society, for the individual's state of mind or karma, or even for the venerable?

I imagine one benefit may be orderliness in the classroom: giving the venerable an opportunity to speak; another benefit is that it might be somehow associated with &quot;faith&quot; (i.e. being willing to listen without yet knowing); is another obedience for some good reason?
The whole question seems to me a bit associated with some identity-view and so thicket-of-views.
The only  answer I can think of is a reference to the sutta (reference required) where the Buddha said that people need some teacher or leader, and he (having none) would take the Dhamma as his -- but that's speculation, whereas I'm asking for answers based on references or experience.
Junior monks have to greet senior monks as "venerable sir" and lay people have to greet monks as "venerable sir" (and not as "friend" or by name) because this is part of the culture of respect and reverence in Buddhism, and South Asian culture in general, both in ancient and modern times. 

It is part of the whole package of reverence to parents, teachers and elders. Of course, in the modern day, this is changing due to exposure to culture of other parts of the world.

Below are some examples from suttas and vinaya:

In AN10.16:


  “Bhikkhus, these ten persons are worthy of gifts, worthy of
  hospitality, worthy of offerings, worthy of reverential salutation, an
  unsurpassed field of merit for the world. What ten? The Tathāgata, the
  Arahant, the Perfectly Enlightened One; a paccekabuddha; the one
  liberated in both respects; the one liberated by wisdom; the body
  witness; the one attained to view; the one liberated by faith; the
  Dhamma follower; the faith follower; and the clan member. These ten
  persons are worthy of gifts, worthy of hospitality, worthy of
  offerings, worthy of reverential salutation, an unsurpassed field of
  merit for the world.”


From AN8.59:


  Monks, there are these eight individuals who are worthy of gifts,
  worthy of hospitality, worthy of offerings, worthy of reverential
  salutation, the unsurpassed field of merit for the world. Which eight?
  
  The one who has entered the stream, the one who has entered upon the
  course for the realization of the fruit of stream-entry, the
  once-returner, the one who has entered upon the course for the
  realization of the fruit of once-returning, the non-returner, the one
  who has entered upon the course for the realization of the fruit of
  non-returning, the arahant, the one who has entered upon the course
  for arahantship


In AN5.199, the Buddha taught:


  "On the occasion when a virtuous person who has gone forth approaches
  a family, the people rise up to greet him, bow down, give him/her a
  seat. On that occasion the family is practicing the practice leading
  to birth in a high family.


In Mangala Sutta:


  "Not to associate with the foolish, but to associate with the wise;
  and to honor those who are worthy of honor — this is the greatest
  blessing.


In AN 4.32: 


  "There are these four grounds for the bonds of fellowship. Which four?
  Generosity, kind words, beneficial help, consistency. These are the
  four grounds for the bonds of fellowship."
  
  Generosity, kind words, beneficial help, &amp; consistency in the face of
  events, in line with what's appropriate in each case, each case. These
  bonds of fellowship [function] in the world like the linchpin in a
  moving cart.
  
  Now, if these bonds of fellowship were lacking, a mother would not
  receive the honor &amp; respect owed by her child, nor would a father
  receive what his child owes him. But because the wise show regard for
  these bonds of fellowship, they achieve greatness and are praised.


In AN5.58:


  Now on that occasion a number of Licchavi youths had taken their
  strung bows and were walking and wandering in the Great Wood,
  accompanied by a pack of dogs, when they saw the Blessed One seated at
  the foot of a tree to dwell for the day. When they saw him, they put
  down their strung bows, sent the dogs off to one side, and approached
  him. They paid homage to the Blessed One and silently stood in
  attendance upon him with their hands joined in reverential salutation.


In MN86:


  "Great king, suppose you were to see Angulimala with his hair &amp; beard
  shaved off, wearing the ochre robe, having gone forth from the home
  life into homelessness, refraining from killing living beings,
  refraining from taking what is not given, refraining from telling
  lies, living the holy life on one meal a day, virtuous &amp; of fine
  character: what would you do to him?"
  
  "We would bow down to him, lord, or rise up to greet him, or offer him
  a seat, or offer him robes, almsfood, lodgings, or medicinal
  requisites for curing illness; or we would arrange a lawful guard,
  protection, &amp; defense.


There are numerous examples of prostration before the Buddha, where the translations say that somebody bowed down then sat to one side.

In MN 90, we see prostrations with head placed at the feet.


  King Pasenadi Kosala said to one of his men, "Come, my good man. Go to
  the Blessed One and, on arrival, showing reverence with your head to
  his feet in my name, ask whether he is free from illness &amp; affliction,
  is carefree, strong, &amp; living in comfort, saying: 'King Pasenadi
  Kosala, lord, shows reverence with his head to your feet and asks
  whether you are free from illness &amp; affliction, are carefree, strong,
  &amp; living in comfort.' And then say: 'Lord, today King Pasenadi Kosala
  will come to see the Blessed One after his morning meal.'"


And also in SN22.87:


  “Well then, friends, pay homage to the Blessed One in my name with
  your head at his feet and say: ‘Venerable sir, the bhikkhu Vakkali is
  sick, afflicted, gravely ill; he pays homage to the Blessed One with
  his head at his feet.’


Part of the Sekhiya rules of the vinaya on a bhikkhu teaching the dhamma:


  When his listener is not ill, a bhikkhu should not teach Dhamma if the
  listener:
  
  
  has an umbrella, staff, knife, weapon in his/her hand.
  is wearing shoes, boots or sandals.
  is sitting in a vehicle when the bhikkhu is in a lower vehicle or not in a vehicle at all.
  is lying down when the bhikkhu is sitting or standing.
  is sitting holding his/her knees.
  is wearing a hat or a turban, or has covered his/her head with a scarf or shawl.
  is sitting on a seat while the bhikkhu is sitting on the ground.
  is sitting on a high seat while the bhikkhu is sitting on a lower seat.
  is sitting while the bhikkhu is standing.
  is walking ahead of the bhikkhu.
  is walking on a path while the bhikkhu is walking beside the path. (Sk 57-72)
  


This theme is also found in Tibetan Buddhism, in the "A Brief Teaching on The Bodhisattva’s Garland of Jewels" with translation and commentary by Khenpo Sodargye Rinpoche. This text might apply to other Mahayana and Vajrayana schools too.

From this section:


  Refrain from meaningless chatter,
  And speak only true words at any time.
  Whenever you see your guru or khenpo,
  Offer to serve with devotion and respect. 
  
  Commentary:
  In every moment, refrain from any talk that does not refer to the
  Buddha-dharma or spiritual practice. Never say anything that may cause
  others to slander the guru or the Three Jewels. Some people pay close
  attention to their own speech, and will carefully consider if what
  they are about to say is meaningful or meaningless, and if they find
  that it is meaningless, they will refrain from speaking at all. Others
  however, very much like talking and gossiping about other people’s
  faults. They do not even notice that they are doing it! This can bring
  a great deal of trouble both to themselves and to others. It would be
  better for this type of person to remain silent and avoid committing
  so many negative actions. Our speech should always be true speech,
  meaning that it is in accordance with the truth, is meaningful and
  brings no harm to ourselves or to others. Whenever we meet our root
  guru or khenpo, we must give rise to reverence and respect from the
  depths of our heart, and offer to serve them with respectful devotion.
  Some people might occasionally become caught up in anger or resentment
  towards their guru. This creates very severe negative karma and should
  be remedied with their utmost efforts at sincere confession and
  clarification. Some people do not know how to act when they see their
  guru or khenpo, and so will try to hide or run away. This is not
  right. What is appropriate is to just remain calm when seeing your
  guru, and, from the heart, offer to serve them with devotion and
  respect.


We also see this in the Manusmrti or Manu's Laws, a Hindu legal text, on respect to elders:


  2.119. One must not sit down on a couch or seat which a superior occupies; and he who occupies a couch or seat shall rise to meet a
  (superior), and (afterwards) salute him.
  
  2.120. For the vital airs of a young man mount upwards to leave his body when an elder approaches; but by rising to meet him and saluting
  he recovers them.
  
  2.121. He who habitually salutes and constantly pays reverence to the aged obtains an increase of four (things), (viz.) length of life,
  knowledge, fame, (and) strength.


And this from the Manusmrti on reverence to the teacher / guru:


  2.146. Of him who gives natural birth and him who gives (the knowledge of) the Veda, the giver of the Veda is the more venerable father; for
  the birth for the sake of the Veda (ensures) eternal (rewards) both in
  this (life) and after death.
  
  2.147. Let him consider that (he received) a (mere animal) existence, when his parents begat him through mutual affection, and when he was
  born from the womb (of his mother).
  
  2.148. But that birth which a teacher acquainted with the whole Veda, in accordance with the law, procures for him through the Savitri, is
  real, exempt from age and death.





  OP: I'm not sure I understand "why", though. For example, a venerable is a friend -- or are they not? Is the behaviour (or mode of
  address) mere ritual? Is there said to be a benefit, some purpose,
  some effect: for society, for the individual's state of mind, or even
  for the venerable? I imagine one benefit may be orderliness in the
  classroom: giving the venerable an opportunity to speak; another might
  be somehow associated with "faith" (i.e. being willing to listen
  without yet knowing)


Argument 1: Korean Air Crashes

A theory on this is explored by Malcolm Gladwell in his book Outliers, as summarized here:


  As a closing point to consider the value of the wide use of formal
  speech in Korean, let me add a story regarding Korean Air discussed
  among others by Malcolm Gladwell in his book ``Outliers." Korean Air
  had in the 1990s become one of the world's most dangerous airlines,
  with so many crashes that it was eventually excluded from its airline
  alliance.
  
  The management of Korean Air at that time brought in an American team
  of consultants to analyze the problem and recommend a solution. While
  the Korean management had suggested more mandatory hours of flight
  simulator training for all pilots, the Americans advised them to
  introduce English as the official on-board language and ban the use of
  any Korean communication from the cockpit. Korean Air went ahead with
  this solution and as a result the crashes ceased, the airline's
  reputation was restored and it became an alliance member again. The
  consultants had discovered that the reason for the numerous accidents
  was that co-pilots could not inform captains about mistakes or
  potentially dangerous situations, because they had to use the most
  formal style of speech when addressing the captain and this created a
  barrier, which made criticism almost impossible. Once they used
  English, this "artificial wall" was torn down and teamwork and
  cross-checks became easy.


According to this theory by Malcolm Gladwell, Korean co-pilots spoke in Korean with each other that had a formal style of speech when addressing the captain, and this created an artificial barrier of respect that prevented first officers from directly criticizing their captains.

They were politely making suggestions with hints to the captain, when they should be communicating directly, clearly and bluntly in the case of a flight scenario where timeliness is critical.

On the other hand, in English, they would address each other simply as "you" or "I" instead of using honorifics. This apparently broke down the artificial barrier of respect and contributed to a reduction in crashes.

I would argue the reverse.

The use of honorifics like "bhante" and "ayasma" would create the right context and climate of respect that makes it conducive for teaching and learning the Dhamma.

Argument 2: Courtroom Decorum

Another way to argue this is to use courtroom etiquette or courtroom decorum as explained by these guidelines:


  
  Be Respectful in Your Public Requests, Comments, and Dealings with the Court. Adopt a formal approach that reflects courtesy and respect
  for the authority of the court. Common phrases that are used when
  communicating with the judge include the following: "May it please the
  Court," used as the greeting at the outset of your opening statement
  and your opening argument; "With Your Honor's leave (or permission), I
  would like to, " used when seeking permission to do something; "As the
  court pleases," "Very well, Your Honor," and "So be it, Your Honor" -
  used when you are acceding (consenting) to an unobjectionable oral
  direction or order of the court. "As the Court well knows," used when
  you are getting ready to educate the judge about something s/he
  probably doesn't know. Don't interrupt the judge. Listen to what the
  judge says. The judge has considerable discretionary power that can be
  used to help you or hurt you, even in jury trials where s/he serves
  only as the referee. Something as nebulous as the judge's tone of
  voice in ruling on objections can influence the juror's perceptions of
  your credibility. In jury trials, treat the judge as though s/he was
  the foreperson of the jury. Every judge has quirks. In some locales,
  there are published Judge's Bench Books that describe the
  peculiarities, predilections, and requirements of each local judge.
  These may be useful reading if you are not personally familiar with
  the way a certain judge runs his or her court.
  


Similarly, we should adopt a formal approach that reflects courtesy and respect for the authority of the teacher.

What would happen if courtroom communication was informal and casual? The authority of the court and the judge would not be respected, and justice would not be served.

Similarly, we need to be formal in front of the Dhamma teacher to respect his authority.

This establishes the atmosphere conducive to learning and teaching the Dhamma.

Argument 3: Your child is not your friend

From a parenting website:


  There are parents out there who are concerned about being their
  child’s friend. What is challenging is that if you are, first, a
  friend, then when the child is a preteen or teenager, the child will
  not need you as a friend. By then, he or she will have friends their
  own age to listen to. As teens, they need you as a parent, but they
  are not going to tell you that. When you become their friend first,
  parenting becomes difficult to establish. The child may not see you as
  an authority figure, and when you try to establish authority, the
  child will most likely question you even more. That’s not what you
  want.
  
  When you are a friend first, it sends a message that you want your
  child to like you, to share with you, and to help you feel connected.
  If that’s the case, that puts a lot more pressure on the child. It is
  not your child’s job to help you feel good about yourself. If that is
  why you are your child’s friend, rather than being his/her parent,
  then you may need to get some counseling for yourself. You and your
  spouse (if married) will need to focus on a healthy relationship so
  the lines of parenting and friendship, even with your child, can be
  reassessed and altered, if need be.


If parents become friends first with their child, their child would not respect the authority of the parent.

It is the same with the Dhamma teacher.

Hence formal communication is needed to establish authority.