A line from Ghost Busters comes to mind:
"Don't cross the streams."
I'm a little indifferent toward Abrams. There's lots to like about him (he amps up the fun, he has a knack for hiring talented actors and/or is great at directing them), but plenty to dislike (can be gimmicky, story telling is lazy, his 'monsters' and effects always come across as blatant - sometimes even lowsy - CGI and rather generic, etc.).
Ultimately, though, this all boils down to the fact that directing a Star Wars movie and directing two Star Trek films is like crossing the streams, particularly given that they'll be almost back to back to back.
It just shouldn't be done, and now we'll all have to worry about the Stay Puft Marshmallow Man.
Ben Affleck was apparently choice #2. Man, we all missed out.