I can sympathise with parents who are at the end of their tether, particularly in public. I had brothers much younger than me (10 and 16 years younger, respectively) and was often left to babysit the oldest one in my early teens. I am not proud of it, but I would often smack him when he was bad, then threaten him not to tell mum. I guess I was immature and didn't know how to handle the situation.
But I will never forget this one time my brother was about 5, and he'd done something to piss me off... and I had him cornered in the toilet, where he'd tried to escape.... I was stronger than him and was able to force the door open before he locked it. I had my arm raised and an expression of absolute fury on my face. And then I stopped and looked at him. The little bugger was scared SHITLESS. He'd gone all pale and his eyes were like dinner plates, and he was cowering in the corner, protecting his head with his arms (I never hit him on the head, btw, only legs). And I just stopped and sunk down the wall and bawled. I felt SO bad. I was meant to be his big sister, his protector... and I saw nothing but pure hate and fear in his eyes. From that day on, I never raised a finger to him. And I never will again, to ANY child.
There *are* better ways. There has to be. Sure, a belting might be effective, but what sort of psychological damage is that doing? My mother has told me numerous times she was "belted black and blue" by her father, and she HATED him for it. She hated his guts. Kids are small and weak and defenceless; and if we use the only power we have over them (physical strength), we are taking the easy way out, in my opinon.
It is selfish and lazy parenting.
I really don't know how I'll discipline my children when I have them, but I know I won't smack. It takes some skill and perseverance to come up with better alternatives, but they have to be out there. Physical discipline is just convenient and quick, it's not necessarily the best way.
That said, I don't judge parents who do it, or have done it, for the very reasons I've written above. I know the frustration and rage that can drive you to it; not to mention the added embarrasment that a public tantrum could cause. But you need to be able to take ownership of your own discomfort and not take it out on your children. And I think with a bit of 'prep work' beforehand and a sense of respect on the part of the child for the adult, it could be avoided.
I was never hit as a child, although my mum often took out her frustration on pillows, and (once) a wall
She was determined never to follow in the path of her father, and I respected her for that. Emotional manipulation worked far better anyway :D (ie. she'd cry, I'd crumble).
I'm sure we all try to do the best we can by our kids. But my opinion on this subject has been affected greatly by that experience I had when I was 15, and I'll never be the same again.