I think the thing to clear up here is that there is a clear distinction between smacking as a form of discipline and snacking out of frustration/anger.
You wanted someone to rationally explain why they smack so here is:
I have 2 DC. DC1 is a usual child. DC2 however is a child like no other I've come across. Just doesn't understand danger. At all. I can explain until I'm blue in the face, prevent and supervise as best I can, but what about those times when I can't supervise properly (in a class of 30 at school, the teacher is distracted and he decides to find out what would happen if he jams a paperclip into a plug socket?).
What I'm saying is I would rather give him a controlled smack than risk him coming to worse harm.
I am a trained childcarer. I have read all the books and know all the techniques, I even advise others on them! None, and I mean NONE, have worked for DC2. For me smacking is not lazy parenting, it is a last resort to keep my children, and your children (I'll come on to that), safe.
Sometimes, to explain the severity of DC2s actions I smack to show him that what he is doing to others hurts them.
I have tried to explain that eg. laying on other children so that they can breathe, is wrong. He just does not get it!! I supervise and try not to put him in situations when it could occur (but to be honest it would mean I would never leave the house or socialise with anyone and that does myself DC1 and DC2 more harm than good).
A smack seems to be the only thing that gets the message through to him. If it was your child he was about to hurt because I'd just tried to reason with him instead of doing EVERYTHING I knew I could to keep others safe, are you honestly saying you would rather I didn't?!
My smacks are not done out of anger or frustration. I calmly, with eye contact, explain to him that his action was unacceptable, and is not allowed. Then I tell him because of this he will be getting a smack. Then I calmly smack him, reiterate why he can't do that thing. And give him a big cuddle and tell him I love him.
He will never recoil if I lunge for him as I explain and warn, never lash out. He isn't scared he will be hit out of the blue.
And this is how I can be a childcarer. I will never hit another person's child because I don't do it as a result of being frustrated or at the end of my tether, it is purely a method I use with my DC.
I absolutely support smacking becoming illegal. There are more people who smack out of anger than do it for discipline properly. If making it illegal stops even some of those people then I am ALL for it.
I would also like to draw a few parallels, play devils advocate, possibly.
Smoking around children causes them physical harm. It also has no benefit for the child or the wider society. It is not illegal and unfortunately plenty of people do it, but it isn't seen as bad as smacking. Why?
Adults can be physically restrained if they are a danger to themselves or others. While I would not restrain a child in this way to discipline them I support this use for adults. Even the mentaly vulnerable. As sometimes it is the only way to keep them/others safe.
What about the military? They are encouraged to fight and kill others, others with less resources/weaker than them. Even kill children who have been trained as soldiers. Why is this violence acceptable? Because it is for the greater good? From whose perspective? The military use violence to keep their families and country safe. I use violence to keep my child and others safe.
I hope this helps you see a different side to smacking. And that not everyone who smacks is "scum", or "a bad parent". As I started my post, there is a major difference between controlled smacking as discipline and lashing out and hitting your children, I agree with you all that the latter is of course totally wrong.