Yes, and what is the likelihood you're going to be able to get a head shot on the first try every time? Now take that likelihood, and add into it movement, changing/unknown distances, and the fact you're probably going to be getting shot back at. Chances are you're not going to be able to do that beautiful little "shot placement" gig you're so proud of, so you might as well be shooting something that they're going to feel no matter where it hits them.
Of course there is the theory of over-penetration and the collateral damage that can accompany it, but that's another discussion altogether.