Interesting question. I've been debating whether to send all my important Pokemon over to Platinum from Pearl. I'd like to keep Pearl as my primary game because of all the accomplishments. Like the globe in the GTS that's filled with locations. And my nearly completed Pokedex. And my underground Secret Base that's set up exactly the way I want it. Minor things, yes, but they contribute to my personal feeling of "completion." I really can't be bothered to try to fill up the GTS globe again, since the entire GTS is messed up with everyone wanting non-event Shaymin and Darkrai.
Look for me to keep active on both games, and maybe, actually, (0_O) someday leaving Platinum behind once the novelty wears off.