Well, the reason it's called medivac is that if danger approaches, it can just pick up all the marines and fly away. The units are healed inside. Besides, medics are about as vurnerable to ground as medivacs are against air, at least with some good micro from the opponent.
I am gonna miss the medic though, remember once when i played against a friend and i didn't want to block the exit to my base with bunkers (so that i could spam out marines), so instead i put up a line of medics and told them to hold ground. When he attacked them with his zerglings they would just heal eachother, and i could just safely stand with my marines behind and pick them off.