They're just playing with the dynamic from TOS season one where the two leads were Kirk and Spock and the two supporting leads were McCoy and Rand. In the show the dynamic of the triumvirate worked really well while Rand's role was limited because the women were largely decorative in the sixties, Kirk needed to play the field, and some other stuff going on behind the scenes led to the actress being fired. In an action movie franchise it's easy to see why Uhura's role as the girlfriend allows her to take up a niche that McCoy can't fill (and Rand was unable to fill because her relationship with Kirk was unrequited). I find it more concerning that Uhura is the only woman but at least Marcus will add to the mix.