What if I only want to ask a two word question like What Joke?
The limit is meant to discourage very short comments like "lol", "thanks", "+1" and so on which are just "noise" of no particular value.
Of course, if you have a valuable comment to post which is that short, it's still easy to work around the limit by adding spaces or dots and even editing the comment to remove the padding after posting it. The limit is not meant to be a bulletproof defense, it's just meant to discourage.
Maybe this is just me, but I've never wanted to post a comment shorter than 15 characters in the first place. It's the maximum comment length that I run into at least a few times a week.