Hunter6 said:
You have two universes-
1.) The Trek A Universe which is the Universe of all past Trek (TOS, TNG,DS9, VOY, ENT and the past 10 movies).
2.) The Trek B Universe which is the JJ Abrams Star Trek Universe.
Now, Nero is not really travels back in time in the Trek A Universe time-line. Nero is leaves the Trek A Universe and goes to the Trek B Universe. Trek B Universe is an alternate timeline which was made by own Nero actions. If Elder Spock leaves Trek A Universe to stop Nero actions in Trek B Universe then Elder Spock is damaging the time-line of Trek B Universe which was made by Nero actions. This is some thing that a character like the Elder Spock would think of. Also a character like the Elder Spock would know about the Many Worlds theory. The Many Worlds theory is the theory that anything thing we do, there are alternate action Universes and there uncountable alternate action Universes. Trek B Universe would just be one other alternate action Universes. The Elder Spock going to Trek B Universe and stopping Nero would be like going to the Mirror Universe and changing the history. A character like The Elder Spock would know that traveling to Trek B Universe is a bad idea, specially seeing like Bob Orci said that his own Trek A Universe continues normally.
I don't know anything about the story line, but what I am getting from your post is that Nero is the bad guy, he goes back in time to screw things up, and so Spock goes back to stop him. Even if it is a different time line that will not mess with time line A, it is still filled with real people whose lives may be adversely effected by Nero's actions, where if Spock can undo what Nero has done and stop him from doing more damage, then the time line should continue pretty much like it did in time line A. But if Nero is left to do as he pleases, then there is no telling what kind of a nightmare universe he could shift time line B into.
Hmm, didn't Voyager and Enterprise both suggest that there was some secret intergalactic unit that policed time and space to prevent stuff like this from happening? I guess that too is out the window.