In an interview, Nimoy said the difference between this cameo and the one he turned down in GEN was that in GEN the lines could've been given to any character (and basically were). In STID, he believed the story lent itself to the idea that Spock would contact Spock Prime about Khan.
His take on the necessity of the scene was that it cleared up any ambiguity for Spock about what kind of person they were dealing with. Hearing Spock Prime's personal experience made it known without a doubt that Spock was dealing with a maniac.
By the way, Nimoy also says he believes Abrams picked up "Star Trek" and elevated it, and he's grateful for that. If Abrams ever phoned him again, he'd take the call.
Here's the link to the entire interview: