"I've never been trained or told to shoot at extremeties, but I've read of weapons and/or hands and arms holding them being unintentionally hit due to the shooters focus on the weapon vs the attacker."
I tend to agree with this. If you are not deliberate about aiming you tend to automatically point where you are looking. Kind of like riding a motorcycle or driving a car out of a skid. Look where you want to go not the direction you are going.
I once had an old cowboy, that was a bit of a quick draw, teach me that if you can learn to point a gun the way you point your finger you will mostly be on target. Try it. Quickly point your finger at something. Then sight down your finger. Most people will be real close to on target.