Ktulu & everyone else:
Please understand that there is a difference between a fragmenting bullet, and a frangible bullet.
Fragmenting bullets will still go through walls and steel, and retain a great deal of energy on the other side, although .223 will have less energy after that than say, 9mm or .45 (according to the .223 faq)
Frangibles, otoh, are designed to pretty much splatter when hitting the first thing in their path.
So yes, lanes of fire are still important to using .223 defensively.