Sadly, altho' it may be at odds with my points of view on 'adult' discipline, I cannot say that I agree with the basic tenet that physical punishment is less than optimal for the training of children.
One of the chief underlying causes for the observed rise of disruptive and criminal behaviour in general society is a lack of proper discipline. The key is in the word really. 'Discipline' is not 'abuse'.
Speaking from my own experience (and there is little else we can all really 'quote' authoratively), not many days went by in my early years when I didn't get a spanking for something. Did this mean I was abused? Not to my mind. I was willful, disobedient and aggressive - I got a good hiding because I would not do what I was told and would not be amenable to reason.
As I was the firstborn and the only boy as well, my father feels he was too hard on me because he was learning how to be a parent. I have to confess that it tears me up that he feels guilty for making me a useful member of society rather than a 'self serving thug'.
I was never under any illusions as to
why I was being punished and from what I can recall I deserved pretty much all instances (other than those cases when my sisters dropped me in it for something they did

.
Physical punishment is a necessary evil for certain personality types, as my upbringing proved, as children are not 'little adults' and nor are they 'innocents'.
They have all the negative and violent instincts of their developing brains and emotions to draw on but none of the trained restraints and cut-outs that you need to function as a societal being. If these impulses are not chanelled and directed then what you get is what you see on the news these days.
For some children, physical punishment is not required it is true. To apply it then would be 'abuse' as they do not need it to adjust their behaviour. For some, whose aggressive nature is like mine was, it is necessary.