I don't really fall on either side of the whole female protagonist thing. On the one hand, saying that the three guys had to be male because that would sell better or because that's more 'realistic' for a crime game is rubbish. It's GTA, they can do with it what they like and I seriously don't think adding a woman would affect the games gigantic sales. Plus, we can always use more strong female protagonists now that Samus has been 'Other M'd', FemShep and the Female Boss' series are over and Jade, Faith and Alyx Vance are still on the horizon.
On the other hand, this is the story the writers wanted to tell and the characters they wanted to tell it with. I would prefer a (relatively) undiverse character set that's written well and written with integrity over something the writers felt obligated to throw in. I like Michael, Franklin & Trevor (though I wish Mike would get over himself once in a while), and am happy enough with them as protagonists that I don't think they needed a fourth member, however diverse he or she may be.