The main reason I suggested it in the first place is that it allows some explanation of avatars. Quite often the only way of finding out what an avatar represents is doing right-click, save as and hope the file name gives it away.
Plus, the option of a bit of comedy gold increases.
I realise that there would probably be a little bit of extra moderation involved, but I'm pretty sure that these can be disallowed in the Mod CP (Admin CP?) in the same way that a signature can.