KDubya: When a mechanic's completely counter-intuitive, overcoming it doesn't feel satisfying, just irritating.
Exploiting... I mean, just respawn the killed parts of enemy stack, explain it away with them also having an endurance system, and heal the damaged monsters. You don't get XP for killing them anyway. I guess it would mean removal of gibs, but eh.
And no, having team mates permanently killed would most certainly not be unduly harsh on casual players. As it stands, when you get into a situation OP is describing, said casual player will have to reload anyway. Some sort of run away mechanic would just give him another choice.
As for cheapening Iron Man - first of all, the mere fact that you can't flee feels like an extremely cheap cop out from "kill a monster - flee" exploits (not that you can't do them anyway in big enough areas, it makes even less sense). Second, such a mechanic would add more options to all difficulties, I've already seen a few people complaining about not being able to run away from the bear.