Devil's advocate.
My siblings and I were occasionally smacked. We have all grown up into gentle, well-behaved citizens too. Just saying because it does not follow that an occasional sharp, even physical reprimand will cause a person to be ungentle and not well-behaved.
No, I am not advocating smacking children, just arguing an empirically proved point.
Other animals (lions, dogs, domestic cats) cuff their offspring when the offspring get out of line.
No, I am not advocating cuffing children, but I don't think it is an all evil, all wicked, all unreasonable thing for a worn out, frustrated parent to resort to. Yes, better parenting techniques exist but not everyone has these at their command at all times.
nankate, asked if anyone else ever felt tempted. Yup, sure did, though actually, kids who are used to reasonableness and rationality find a sharp No! pretty effective. They have to have learned that when you say No! you mean it though ?