Some kids are more stubborn or willful and will not respond to, say, time outs or revoking of privileges. Studies suggest that siblings tend to be about as alike to one another regardless of whether or not they were raised in the same house or by the same set of parents, suggesting that parents have less to do with who we are than might intuitively seem to be the case. That being the case, it's not at all impossible that an otherwise good parent might be brought to use physical punishment in some circumstances. That there are many children who grow into adulthood without having been spanked does not mean that no individual child can benefit more from spanking than from non-corporal punishment, nor does it imply that children who ARE spanked necessarily turn out worse. Some will, of course, perhaps disproportionately many, if currently sociological studies are to be believed (though such studies have been quite criticized over the years), but that doesn't mean that there aren't kids who won't turn out better from spanking or that there aren't kids who weren't spanked who might have turned out better had they been spanked. Even if one is opposed to spanking, I find the conflation of the smacking of the buttocks with "physical assault" more than just a little hyperbolic and disingenuous. I was not spanked, perhaps would not spank my kids if I had any, but to judge others' whose situations you do not (and likely cannot) know the specifics and nuances of IS a form of sciolism, even if well-intentioned.
Edit: And yeah, the whole "You're teaching your kid that violence is how you get what you want!" BS is tiring. No, you're teaching the kids that actions have consequences, the possibility of pain among them.