there's too many reasons not to have an AR pistol in 5.56, most have been stated already. The 5.56 AR in a small package gives you more range & power than a 9mm version if you need to shoot farther, your taken care of. The ammo cost is not much more & as others have said its a common caliber in a SHTF scenario. Rifes the 5.56 is a do all out to 300-400 yards. The 7.62x39 has more lead but still lacks the range of other options in the AR15 platform. If your looking for 308 range in a AR15 platform theres the 6.5 grendel that has great ballistics out to 800+ yards.
In the end, its all a matter of your money & choice to get whats best for your needs.
In the end, its all a matter of your money & choice to get whats best for your needs.