I am not a full time portrait shooter (ironically, we are doing some this evening), but I look at it this way.
I want as much as a camera can give me, and then it's left up to me to provide whatever limits I want. So I would take a 50Mpx image any time for portraits, and then I can resample down as needed, I can post process some things out in the large resolution, and then after resampling, the results are even better, etc.
It is easier to shrink what a camera can produce down than it is to make something out of nothing later with a less capable camera.
In other words, I want to be the limit to my results, not my gear, resolution, fine detail, no matter what.