How does realism factor into a video game RPG? What's the use of things like HP and leveling if everything to the head is going to mean instant death. And unless there's an auto save that saves every second no one would like an actual realistic game. Well... unless that game was called hospital and you played a doctor trying to save characters who had sustained wounds. You get points for every patient you save. Best of all the game ties into lots of war games and first person shooter.. if your character gets shot you'll be whisked away to the hospital game where hopefully you get a really good player who can heal you and send you back into the other game.
That's not true. A lot of people love "realistic games". Arma for instance. You don't need an auto save that saves every second, you just try to stay alive, and enjoy the challenge and realism. It adds to the immersion and fun.
However, I don't think any kind of "One hit kills" suit a role playing game. I would, however, enjoy it if a shot to the head cause more HP loss. It adds to the RP elements.