If the Federation use Spock's knowledge of the future to its benefit, why the hell wouldn't the Klingons send their own emissary back earlier still to undo him and do even more to its own benefit. Same with the Romulans, Tholians, or Borg?
With Spock's knowledge the Federation wouldn't merely deal with the space amoeba and save a redshirt or fifty, but they could take over the galaxy. THAT's why the JJ-verse won't have him spill too much of what he knows: because it wouldn't be about giving the writers room for new developments in the Trek universe, it'd be them having to create a Trek universe too
divergent from what they're looking for.