Creative incest this would have been.
Having the same director for both Trek and Wars is already enough. Do they also have to share the actors? That's silly.
There are 100,000 unemployed actors in Hollywood. Give other people a chance.
Abrams, like many directors, has a thing with working with the same actors. Just from the Trek cast, Simon Pegg is in Bad Robot's Mission Impossible movies and Karl Urban is in that new Almost Human show. Add onto that all the actors who have been in multiple Abrams projects.
It's nothing unusual. Christopher Nolan casts Michael Caine in everything, has worked with Tom Hardy and Christian Bale multiple times. Joss Whedon is constantly bringing his cadre of actors on his many projects. And so on.
Basically, there's no reason we can't expect a few Abrams Trek stars to show up in Star Wars Episode 7. I'm sure Simon Pegg would like to be able to say he's been in all three of the world's biggest sci-fi franchises. It'll sure be surreal though, since very few actors have been involved in both Trek and Wars. Leaving out voice actors in Clone Wars, I doubt the number of actors to have roles in the live action movies/episodes goes above ten.