There are turtles, rabbits, different kinds of fish (labeled when I look at them!), even insects in the game. All of them not necessary for a military simulator! And nobody crys about BI spending effort on that. But still they took the time and effort to make them, to make the game more realistic and diverse. For the very same reason there should be playable women in the game. (The realism part has been covered extensively in this thread already, there are plenty of women in the military.)
You don't need different skincolors or faces in the game yet they were an option in Arma 2, there even was a ridiculous amount of them all with different names. If they have time for that there surely must be time for one or two female heads. The model doesn't need to be changed that much, just change a few poligons. Animations can stay the same, too, there's not that much of a difference. And as I have said before, they are already creating all of that for DayZ, they can just use the same data for Arma 3, not much extra work required!
And for those afraid of naked women running around: I expect that the unclothed female model would be wearing a tshirt and covering underpants just like the male model. Even if there won't be female models there will probably someone be out there stupid enough to run around with a male "naked" model because they think it's funny. No way to avoid that.