Spock can probably alert people of certain highly specific events that are likely to still happen- the Doomsday Machine is going to be in X place at X time - a certain colony is going to be infested with flying fried eggs - a certain planet is shaping itself into a simulcrum of Chicago in the '20 - but the timeline changes have upended so much of the political landscape that big-picture stuff that empires would be interested is exactly the kind of stuff Spock can't help anyone with - he can tell them how things would have turned out, if Nero hadn't thrown everything for a loop, but they're still going to have to figure most of it out for themselves.
The problem is, how do you know what events are still going to happen and what events have now gone poof? The more isolated the event is from other stuff, the more likely it is to remain the same as Spock remebers - like the Doomsday Machine which is still way the hell out there somewhere, or the Borg are examples of probably unaffected stuff. Knowing about those things aren't going to help the Klingons and anyway Spock will have already told the Federation about it, so Klingons can't decide, say, to corral the Doomsday Machine and use it as a weapon since the Feds are already on their way to blow it the hell up (if for no other reason than to make sure nobody can use it as a weapon).
