The problem with all health systems so far is that they don't really imitate how a real person gets injured and recovers. Eg if I jam my finger in the door, it technically 'injures' me (bruise and pain), however I could jam my finger a million times, and I wouldn't die. However, on a game, ANYTHING that damages you can generally kill you if it happens enough times. Also, if I get injured in real life to a certain extent, I will either heal very very slowly, or slowly get more injured, depending on what happened. Games rarely take that into account.
A realistic damage model would be incredilbly impractical: Who wants to hide in a corner for five weeks while your character's broken leg heals?