I disagree. His cameo was essential to nuSpock's next actions.
Not really. Sure, the scene was structured so that Spock Prime's information helped Young Spock make his decision. But if Nimoy hadn't agreed to the cameo, it would've been simple enough to rewrite the script so that Young Spock came to that decision on his own.
As for why Nimoy agreed to this cameo but not the one in GEN, it could be largely because Nimoy's evidently developed a good working relationship with Abrams. Abrams has managed to coax him out of retirement a couple of times, first on ST'09, then on Fringe
. So clearly he likes working with the guy. And that would've made it easier for Abrams to convince him to do a quick scene for STID.
Also, since his scene was just on the viewscreen, he didn't have to go to the trouble of coming to the set and waiting around for hours for various shots to be set up and multiple takes shot and so on. He just had to come in, put on the makeup and wardrobe, sit in front of a backdrop and read a few lines, then go home. It was much less of a commitment of his time and effort.