In a military role, the 5.56 works well. Even though it may not be a fast "stopper" usually once wounded, an enemy combatant is out of the fight. Several others are tied up transporting and treating the wounded man, tying up enemy recources.
In a civillian role, as in self defense, usually you want to stop your enemy as fast as possible, before he can finish what ever he was doing that caused you to use deadly force in the first place. That means stopping power - expanding bullets and deep penitration. In my AR platform rifles I use for possible self defense, I keep them loaded with Barnes X bullets, the bullets designed to make the 5.56 a deer gun. I figure this would give me a good combination of expansion and penitration, or "stopping power". For what its worth, I have seen 5.56, 55 grain ball used twice on suspects by the agency I work for. One man dropped immediately, the other ran about 50 yards, climbed a fence, then gave up. Both were body hits, both survived.
Larry