The cold hard truth is you arent gonna carry both a 5.56 and 300 BLK upper and ammo with you if you ever "have" to use them. Looks like there isn't really any advantage.
You're comparing the 5.56 to the .300blk. The advantage comes in .300blk vs .308win or 7.62x39.
For someone who wants .308 caliber projectiles in an AR platform, the .300blk is a killer option.