It's always nice to see Nimoy again. I think he wanted to appear but I'm going to guess he probably has health problems and probably can't appear "live" for too long. I've noticed he had very short parts or 'phoned in' scenes in the latter parts of Fringe also.
I'm going with "like" on this. I think this should probably be the last Spock cameo, however. I'm sure the next movie will feature a cameo from someone else (my wild guess is going to be Stewart).