In the fleet, the MC is the senior man in the crew. Can be the pilot or ECMO. I was the third most senior (lineal) guy in my squadron so I was usually the MC unless flying with the CO or XO. We had JO MC's and sent out a couple of cross countries that way.
Front seat (ECMO1) time however was very important and usually tracked by scheds or decided in the crew brief. A MC ECMO could decide to sit in the front more than normal but he would be called on it by everyone else. If the pilot happens to be a MC, we usually let him fly the plane anyway. :icon_wink