Sometimes you don't have time to realign your shot to a more effective place. By the time you do that you'd probably be already dead. I've yet to see a war where every casualty was a headshot. And the idea itself isn't attributed to John Wayne...
RTFP. That's not what I said at all. If being shot in the arm was made "lethal" (read: makes you combat ineffective) than that shot would KILL them, as opposed to making them drop their gun like a dope and keep coming at you with their fists or stop and try to pick it up.
Even if it is a "one-in-a-million lucky shot", it would be rewarding if you got it, no? Especially when you're suddenly rushed and had to resort to hip-firing.
No, it wouldn't be. It'd be stupid, unrealistic, and frustrating to have to prime the bolt again, or pump another pistol round into his chest to get him to drop. Even worse is if he kills me after he SHOULD have been rendered combat ineffective, but instead simply lost his gun like a bad western and kept coming at me.