It's a fine line, and difficult to define. If your intent is to teach rather than to punish, you won't, or at least will very rarely, run into a point where corporal punishment becomes a necessity in disciplining a child. I think the main issue is that beatings for everything do not communicate consequences as a direct result of their actions, the goal of any discipline. Beatings only communicate that violating your whims results in violence. There's no connection with their actions, and no lesson learned. Therefore, they will grow up believing that one day it will be their turn to be the adult, and their whims will rule over others.
Essentially, beating your kid because he tried to run into traffic means he does not learn that running into traffic results in danger because cars are dangerous, but instead that offending my dad results in beatings, because offending people bigger than me means they have an excuse to beat me.
That said, if I saw a parent using force to discipline a child in a way that wasn't going to cause injury (a spanking, etc) I would assume that, barring any other obvious indicators, that they have run out of other options.
Violence is also something that carries an instinctive negative meaning. I have had instances where I have smacked my kid on the hand with so little force that it would not break an egg, but the act itself carried enough meaning that the message got through where words alone were not. No tears cried or marks left on child, but the message that
this was not ok was received.
I knew a chick who fed her 3-year-old potato chips for dinner along with her twice-daily energy drink mix.
She was, uh... A strange lady. No idea how the kid's doing these days.