.223 is enough bullet 80+% of the time, but (I believe) we're talking about a situation where your very life depends on your rifle being able to handle ANY situation. Bears were mentioned. If all you have is a .223, don't bother. Someone hiding in a field of grass/weeds? .223 bullets are light enough to be deflected easily.
Assuming you have a 20" AR and you're using M193, the max range that you'll get reliable bullet fragmentation is 200 yards. This is what gives the rifle "stopping power."** If you have a shorter barrel, or you're using underloaded ammo, or using M855, the range is less. .308 rifles have 800 yards of stopping power (assuming you can connect), but they can also shoot through stuff that would be "cover" from .223, and still retain enough power to do major damage.
Again, the original question assumed that you'd have one rifle, and be out in the world where you'd need your rifle to get you through any situation, from urban to rural. .223 isn't it. I wish it was, 'cause M16s are a lot lighter than FALs, but it just isn't so.
-Troy