Well, the alternative would be having our medical students practice by operating on animals - puppies and monkeys and such. Leaving aside how you feel about the animals themselves, the question is...would that be more or less desensitizing than practicing on a mannequin?
I'm no psychologist, but I would guess that whatever is MOST traumatic would also be most desensitizing. For instance, if we had them practice - and fail! - on real people, they would probably get shell shocked pretty fast, a lot faster than if they were just using mannequins. So your goal might be to get the best training with the least stress as possible. Something even more removed from real life, like a video game, might be less stressful, but the training provided would also be less valuable. Real animals or people would offer better training, but would stress out our doctors more than we'd like. Yaaay, mannequin!