Is Buddhism a fear-based religion? My general observation about religions is that they have this fundamental assumption of fear of something. For example, fear of punishment or getting into hell is present in many religions. Buddhism too has this fear of rebirth in lower realms, but also a fear of getting into sadness (dukkha?)

These days people of modern outlook tend to discount such fears as childish and practically useless. While a philosophy that distinguishes between harmful and beneficial will still be valid to a general society even after religions disappear.

How can Buddhism be helpful to a secular society that does not have any irrational fear of hell or getting into sadness? Additionally, how can Buddhism be useful to a practical society concerned with real harm and benefit?
According to Theravada Buddhism, fear is based on anger. As per the Visuddhimagga-Tika:


  na bhāyati ñāṇassa bhāyanākārena appavattanato. paṭighacittuppādavasena hi bhāyanaṃ, ñāṇaṃ pana bhāyitabbavatthuṃ bhāyitabbanti yāthāvato jānāti.
  
  It doesn't fear - because of the non-occurrence of the condition of fearing in this knowledge [knowledge of fearsomeness]. For, fearing is through the influence of a mind of aversion, but this knowledge knows, of a thing to be feared, merely: "it is to be feared".
  
  Vism-Tika 21.3


The Visuddhimagga itself (quoted above in bold) provides a good explanation of the difference between knowledge and fear:


  But does the knowledge of appearance as terror [itself] fear or does it not
  fear? It does not fear. For it is simply the mere judgment that past formations have
  ceased, present ones are ceasing, and future ones will cease. Just as a man with
  eyes looking at three charcoal pits at a city gate is not himself afraid, since he
  only forms the mere judgment that all who fall into them will suffer no little
  pain;—or just as when a man with eyes looks at three spikes set in a row, an
  acacia spike, an iron spike, and a gold spike, he is not himself afraid, since he
  only forms the mere judgment that all who fall on these spikes will suffer no
  little pain;—so too the knowledge of appearance as terror does not itself fear; it
  only forms the mere judgment that in the three kinds of becoming, which resemble
  the three charcoal pits and the three spikes, past formations have ceased, present
  ones are ceasing, and future ones will cease.
  
  Path of Purification, XXI.32


The important distinction here is that fear itself is unwholesome, so Buddhist practice is for becoming free from fear. Unfortunately, apart from never being subject to that which is fearsome, the only way to become completely free from fear is to practice so that one is no longer afraid of that which is fearsome (i.e. by freeing oneself from attachment to the objects about which one is afraid).

So, rather than practicing religious practices to avoid situations of which one is afraid, the point of Buddhism is to practice in a way that allows one to remove one's fear of such situations. The premise is the same; namely that one is afraid of certain situations. The difference is in how one relates to this fact. In regards to bhayañāṇa (knowledge of fearsomeness), the expectation is that the knowledge will help one to let go of one's fears, once one realizes that there is no real, lasting escape for one with fear.

The claim that non-religious people can somehow be free from fear without such practice depends in a large part on whether there is indeed life after death; still, it is most certain that those who scoff at the fears of hell and the afterlife would still experience great fear in this life if they were to meet with what is truly fearsome (disaster, danger, etc.) and so they have not really accomplished the goal by simply discarding fear of that which they don't believe to exist.

Update:

As for the criticism that Buddhism uses fear as a means of encouraging proper practice, I suppose this might be true conventionally; a person who does something not knowing the consequences of that act ought to be reminded of the danger. In a conventional sense, that may bring fear to the individual. But the individual need not be afraid, they should simply avoid the activity. 

As to your additional questions, fear of hell is only irrational if there is no hell. Fear is always detrimental, though, and in fact a prime reason for being reborn in hell; only those with minds inclining towards anger will suffer from fear. Buddhism helps one become free from anger and thus fear; whether there is an afterlife or not, this is surely a beneficial thing to modern society.