GIVE THAT MAN A CUPIE DOLL!!
Did he go back in time and save Zarabeth from a cold and lonely death? How about poor I-Chaya? Of course Spock learned a hard lesson about mucking with the timeline in "Yesteryear".
Zarabeth was part of another planet's history that played out as it should. I don't recall I-Chaya, but if Spock knew how it was suppose to go and it was screwed up, he would've done something about it. If he was part of messing up the time, he'd go and fix it. If it meant the saving of a planet, he'd find a way. And come on, Spock's MOM!
If he learned a lesson from "Yesteryear" he clearly didn't apply it in "The Voyage Home" with the planet Earth.
Star Trek XI already has a compromised timeline and arguably one that is in an alternate universe. The only "logical" excuse that our Spock didn't do more is because "this reality is of no consequence because it isn't mine" - borrowed from SG1.