There are two problems that I see with the "Temporal Prime Directive" argument.
1. The events of the new films take place before Starfleet had developed TNG-style rules about changing timelines.
2. The characters in the film themselves
recognized that they were in an alternate timeline caused by Nero's intervention.
Nu-Trek is not part of the old timeline, but is boldly going in its own direction. There is not future to screw up or ruin. They are on a brand new branch in a branching timeline.
Their timeline is already radically different from that of old Trek. They've learned that their technological disadvantages can result in entire planets getting destroyed while their fleets are blasted apart like toys.
Their future is open (that was the whole point of the reboot).
Spock would simply be like encountering an alien with superior technology the UFP with whom the UFP commonly interacts to get more tech. Spock is THE surviving elder. He may save countless lives, prevent countless wars, cure countless diseases.