Cartridges don't kill critters, bullets do. Loaded with a proper bullet, the 5.56 is a wonderful hunting cartridge in it's class.
Most any centerfire rifle could be a poor choice four urban/home defense. But given the proper bullet, the 5.56 can be the preferred choice if a rifle is used.
Most modern body armor is the popular Class III, which is rated to fend off penetration from a 147 grain 30 caliber bullet traveling at 2760 FPS. This implies that many cartridges are ineffectual on body armor.
So, Ruger has an AR with a new finish, some will buy it. I'll bet that's Rugers goal. Otherwise, the chambering is still just fine IMO. 8)