In combat it's more efficient to wound the enemy than to outright kill them. A wounded combatant takes their squad-mates out of the fight as they tend to them.

I've heard that stated both as fact and rebutted as a myth, so I'm not sure if it's correct. I do know the 5.56 was designed for high velocity, and to allow infantry to carry more rounds than the standard 7.62x51.

As for shooting deer, yeah obviously the 5.56 is totally capable of killing deer, but when I'm hunting I don't want something that's 'good enough'. As a hunter you want to shoot the largest caliber that you feel comfortable making a good shot with, so the occasional marginal shot will still result in a quick kill.