Surprisingly it seems no one asked this before.
Not long ago I asked a question on the site, and in a comment I was asked "what version of jQuery are you using?". The answer is obviously less than 15 characters. So I had to do something like this "I am using version 1.9.1" instead of just "1.9.1".
I see this a lot, so why not allow shorter comments?
I guess the idea is to force people to give more thorough comments and not just small quirks, but I think it reasonable to allow users with a ceratin amount of reputation to give shorter comments, because sometimes it just makes more sense and you don't really need 15 characters