For a discussion group I might agree, but SO is supposed to be a repository of legitimate questions and their legitimate answers, not a place for general banter.

nobody is. just the moderators. it's recursive.

I'm fine with it.

It's kind of the same as here on HN. If unconstructive comments weren't discouraged here, the comments section here would look like on Reddit.

If unconstructive answers weren't discouraged on SO, I'm quite sure the more popular questions would be full of flippant half-answers trying to be funny or people posting memes to grab attention.