Some, yes. I <3 the traded exp bonus they have, even though you technically catch them yourself. (btw, catching contest = stupidest minigame, EVER. There's no skill/challenge involved. Disappointing).
No. Once Pokemon are in P/D, they're locked there and forever gone from R/S/E. Kind of a scary thought, but then you figure that you're probably going to be doing 99.9% of your battling on P/D.
I can't imagine all the glitches that could be spawned if you were allowed to send them back to R/S/E. I've already seen some pretty crazy ones played out on YouTube.