Spock also knows of the universe being overwritten by changes to the past (City on the Edge of Forever and probably First Contact.) You would think that if he wanted to minimize his contamination of this new timeline he'd be working to get back to his own. It's not like taking care of 10,000 Vulcans is that big a job. It's a good sized town worth of people, not even a city. Even if there aren't other Vulcan colony worlds (and it doesn't appear that there are otherwise why not settle the survivors on one of them rather than finding a new wold for them to live on) the Federation could probably have a colony up and running in a matter of months if not weeks. All he's doing by sticking around is making himself a target and putting those around him in danger.
Why is I'm the only one who sees the impotance of Kirk and all being in the right spot at the right time to stop Lazarus, The Space Ameoba, the Cloud Monster and The Doomsday Machine? V'Ger too I suppose...and at some point, somebody better start cloning some whales damnit.
You can't just leave some notes and hope it gets done, he should stick around for as long as he can and help.*
*Heh. and the first thing he does is to say "You and that Vulcan guy gotta have a bromance!!" Something the universe doesn't seem to think is a priority. At least nothing I've seen from the trailers indicates so. If anything the universe replaced Kirk with Uhura.