Simple reason to do sigs as they are...you don't have to just put a sig in your node template, you can put other tags you like to use. The other is simplicity, this is done and works, and if it ain't broke, nobody is going to volunteer to fix it ;)

Do not misunderstand me: I thoroughly second that. It's also not a problem for me because I do not have a sig here and I don't want to, although I have several ones for email. Anyway I agree that the sig must become part of the message, unlike in those fora in which it changes everytime in all of what you ever wrote, when you modify it: it's true that one should not comment sigs, but if she does, then her message may risk of losing sense if that mechanism was in place. Yet I'd conceptually prefer it not to appear at all in the composition window, but only to be appended after submitting. It's just me, probably, and as I wrote I don't really mind...