If it is relatively easy to implement and the devs don't spend time on it that they could be spending on stuff with a higher priority, I'm all for dismemberment. As long as it was not too easy to get dismembered it could be really good, both increasing immersion and heightening emotional attachment to your character. You really don't want dismemberment to become too cheap, "Oh, there goes another limb". Also, I think a realistic health model is essential.
Imagine the suspense.
You have just slain an immense and monstrous creature in its lair, but with its final blow, it punctured your lungs. You stagger back to the surface world, fighting death every step of the way. You manage to stow your weapons and mount your horse which was tied off outside the cave. As your horse trots back to a nearby town you start hearing noises in the dark. You urge your horse onward, trying to maintain your seat, wincing with every step it takes. As you arrive in the village, you fall off your horse and stumble to the local place of worship. The cough blood and bone shards as the head healer attempts to restores your former vigour...
Not very well written but I'm sure you can imagine it. Or if their was no local healer, you could try to bring the loot you collected back to your abode before you expired.
Also:
@Rob
Have you played Day-Z? The more stuff I read on this forum, the more this game is starting to sound like it. They use the system of bleeding out and players have to maintain a certain blood level using morphine, food, and bandages to counteract bleeding.
@walltar
If it was a pure medieval setting, I would totally agree with you. But let's not forget about thaumaturgy. I think healing would come under the "Body" magic category, and could be used to treat internal bleeding.