When I was a moderator, dhpackr, I used to edit people's posts to cut out the fat There are times people literally include the ENTIRE thread in their post, only to add a sentence or two. Whereas I trim quotes down to exactly the part I'm responding to, sometimes only a few words from a sentence. The rest of the thread is there for everyone to read; we don't need to rehash it. A quote is often necessary to set a response in context, so only as much as is required to establish the context should be included.
The problem is that most people are on broadband connections with ultra-modern browsers these days. They don't understand how LONG it can take for such cluttered threads to load on slower connections or on computers running antiquated browsers like IE 5 (such as the computers at my school's cyber cafe).
It's also just annoying to have to scroll an entire screen and a half to get to a couple of new words.
But I've been accused repeatedly of making a mountain out of a molehill, so I'll shut up.