I think in the movie Spock Prime gave us a big hint of what he'd do with all his "special" information. He took a gamble (he called it an act of faith) when he chose not to aid in stopping Nero, even though he could've been of great assistance. In this case, he wasn't even holding any future knowledge of events. His intervention could have been justified even with a temporal prime directive. But he opted out.
He risked the fate of billions of people on Earth that Kirk and Spock would succeed without his aid, just so Kirk and Spock could see all that they could accomplish, together.
If he was willing to allow that, then -- .
Spock, as he is written in the film, certainly appeared to be more concerned with saving the bromance, than with saving Earth.
As I, however, am criticizing the presence of Old Spock in the old timeline as a narrative flaw, I am not committed to praising Spock as he is written in this film (indeed, I object to his very presence in the film! - I maintain they should have done a hard reboot). It is, simply, more evidence that the artwork is flawed. I am only committed to conceding Spock's rationality, as he is written in this film, if I am also committed to the overall quality of writing in the film. Since I am raising questions, I am clearly not committed to conceding this.
The only Spock I am committed to (as a normative example - something which sets a proper expectation by which we would judge the film) is Spock of TOS and the films. We cannot judge the film by Spock's assessment of rational choice in the film, because this would be circular reasoning.
And this Spock (TOS) would and did intervene to mend timelines and to make the world a better place. He did it in City on the Edge of Forever. He did in Star Trek IV. etc., etc.