I've always wondered about that, too, but it does seem to bother
some people for some reason. Obviously, to quote someone's words
as your own is clearly deceptive and unfair; but to take a short
excerpt from someone's posting in order to reply is a different
matter altogether. That's just saying "the idea X has been said
by someone, and I think..."
Copyright does not apply. Such excerpts for comment are clearly
fair use under US law.
Attribution does help clear up 3-way discussions, and clarifies
long-running discussions, so I generally include it if it is not
overly burdensome, but I see no reason to be a slave to them, or
to become annoyed when mine are left out. In fact, I rather
prefer when my text is not attributed, as it means people are
thinking about the ideas and not my net-persona.