Marcus being "part of the crew" should please all kinds of folks that have always had a problem with Uhura being the only female in the promary crew.
I would have preferred M'Ress. Not only would she have also added a female crew member to the roster, she'd also be the first full-blooded alien who's part of the Enterprise's crew that has a freaking name. Instead we just get another human crew member to help cement the Klingon's claims that Starfleet is a homo-sapians only club.
WEll, looking at the last 2 movies, those claims would not be correct.
Hell, even the Orion Slave Girls are in Star Fleet now.