I think there is no reason in the world that a .223 should be used on anything bigger than a coyote. Will it work? Sure, but will it also fail sometimes? Probably. If you need a follow up shot insanely fast on a big game hunt, you need to make that first shot more effective.
Just my opinion, CApighunter
It all boils down to freedom of choice. As a matter of common courtesy, I don't bother other people with my opinions about what they choose to hunt with, and expect that to be mutual. Everyone speaks for him- or herself, of course, but when we go to generalizations and narrow our points of view enough for them to make sense on a personal level, something has gone terribly wrong.
I like to view AR:s (and all semiauto rifles, regarless of cosmetics) as the true spiritual successors of big game double rifles that were born out of necessity, a near-instant follow-up shot at dangerous game.
AR-platform hasn't been strictly .223 for a long time. Mine range from .22lr to .308 and I've been looking for an excuse to get a .50 Beowulf upper for a while now.
I really wouldn't worry about what people think. Like, generally speaking, I don't really worry about other issues people have inside their heads, unless when I can bill them by the hour for listening their personal problems. Everyone's got an opinion, but whether it's based on facts, whether it's thought out properly, or whether it makes sense objectively is another matter.