The problem with that is, it's only useful of you're being shot. Incidentally, it's also the best way to get yourself killed in Impossible :PLots of valid points for the heavy and sniper MECs, but what about the support MEC? I was thinking of adding one on Impossible. Great aim+grenades+defense buff means the MEC can just hang back and destroy cover, and move up to buff another unit that is in LOS of a thin man.
I just find that I often get hammered if I give the enemy a turn to act. Thus, I've had far more success with an offensive strategy on Impossible. A support MEC, while good on paper, simply doesn't fit in when you simply want to kill them before they take a shot.