Question: When a new member tries to write a comment he sees “add comment (requires 50 reputation)” but of course he has no so many points. On the other hand, answering does not require any reputation. Then the only option is to mask the comment as an answer, but it may look silly. I think this is not the right policy, as new members may be discouraged to contribute to the discussions, even if they have some interesting and valuable remarks.

(By the way, the same situation is in math.stackexchange.)

Answer: 
There are several related reasons, but a main one is simply **spam protection** An answer bumps and thus it will get visibility. A comment not. If everybody could post comments right away they couuld spam the site (with actual spam or just nonsense) and it might take much longer until it is noticed.

There is also the issu that if you give brand new users the ability to ask, answer, and comment there could be still more confusion for some which is for which type of contribution. 

I see the issue that it can be limiting not to be able to comment, but if the comment feels important give it as an answer (and add that you are doing so because you cannot comment to avoid the 'looking stupid'.) It is now even possible for moderators to turn answers into comments so this is now even less of a problem. (But, please, really only if it is important.)
Well since you advice posting as an answer and saying that it's only a comment I will do it. As many commenters said, for infrequent user that is a quite bad threshold. The issue is in many topics I have a thing to bring to the discussion, but not a real answer.

So I try to post my comment as an answer saying first that I don't have enough reputation to comment. But many times I've been welcome by a "It's not because you apologize and explain that it's right to do the wrong thing". Oh yeah then what? I ask about 12 questions of which I already know the answer to hope and get enough upvotes to get my 50 reps? looks a bit weird.

So I'll get to the point of how it should be changed in my opinion, now tell me if there's anyone I can suggest this to. So many of you said it was spam protection. But that actually is nonsense, since it only requires a 0 rep account to post questions and answers. So if you really want to spam, you can do so by over answering... Sure that will be deleted by down votes and so on, but so will comments. And answers have MUCH more visibility than comments do =/ So if anything, the rep limit should indeed be 50, on that I agree, but not to comment, to answer. This way a spammer will get less visibility, and someone like me that just wants to throw his 2 cents on the subject can do it =/

For example there's a very interesting subject on StackOverflow about a new coding idiom found by a user for which I have found a way to enhance the security but people won't consider it as an answer and I can't comment, so I'm stuck not being able to share what I found :(