Conventional cooking heats by conduction and the outside will always cook more than inside, but microwave energy can provide heating beneath the surface.
Although some say microwaves cook from "inside out", this isn't quite correct. The penetration depth is something around 1.7cm (for residential 2.45GHz model, about 2-3 times deeper for 0.92 GHz industrial microwaves) and the microwave energy is reduced as you increase in depth. Shallower depth heats up more,however microwave heating below the surface as well.
The idea is that microwave can cook the meat with lesser variation in doneness through the full depth. You can further reduce the temperature gradient by cooking at reduced duty cycle (the power setting does not actually reduce the power, but changes the amount of pause in between heating)
I'll either pre-heat the meat in the microwave then finish on the Geroge Foreman grill, or I'll sear both sides on the GF grill first to seal the meat, then finish cooking the meat to desired doneness in the microwave. The surface finish is important in making the meat presentable, so microwave alone won't do it, however used in conjunction with G.F. grill, the outcome is completely fine for me. While microwaving I put the meat on the plate, then put the plate on an inverted bowl, so that microwave energy can access the bottom as well.
What I get is meat that is adequately cooked in the center yet tender all the way to the surface.
Using similar thickness and weight cuts, you can get the cooking time to exact science and get a very consistent result every time.
Any other advocates for microwave assisted steak cooking?