I think Time Travel was the only way to bring Leonard Nimoy into the film. Even if Star Trek started in the 23rd Century, some form of time travel would have to occur if Nimoy was to be a part of it. They really couldn't have a "young" version of Nimoy automatically in the 23rd Century because of his age. Besides, I think it's a good idea. If this movie doesn't make it big, then it would be easy to tell 24th century stories again.
I seem to remember reading somewhere some time ago that Nimoy would play Spock's father in the movie, so that would also have been a way to have a Nimoy cameo. Of course it wouldn't be Spock, but everyone would get the hint. I guess by dragging the 'real Spock' into this movie, you need to either address questions as to what happened to him since TNG's Unification (and thus get entangled in 'the old Trek' again) or risk pissing a lot of people of. Ah well, we'll see.
I don't know if I would of liked that idea. I remember when Back to the Future II came out and Michael J. Fox was to play several parts for his family in the future that I wasn't to crazy about that idea, either. Nobody could play Sarek better then Mark Lenard.