Level 78 green cata quest rewards are higher ilvl than 277 ICC25 hard mode epics. If you're in full 277+ epics, you'll probably start replacing gear with cata blues on day 1.

Your post title is quite misleading. I think there's a very very small minority who only raid so they can gear themselves out to level in the next expansion. In fact, doing dungeons/raids in cata is pointless right now because WoW 5.0 will replace all those items when it comes out! :P

I haven't killed Arthas 25 hard mode yet, so there's still a big reason for me to continue raiding.