I do think lots of parents have smacked at some point (even if a one off) as most of us experience a loss of situation-control or run out of other options at some point. For those who have never done, ever...I applaud you.
We smack as a last resort but as DS1 has just turned 4, I am fast realizing that it isn't as effective as withdrawal (of treats, TV, special outings etc). And I don't like doing it, and yes I do think it can represent a loss of control (although we do it very lightly and there are lots of warnings beforehand..having said that a couple of times I did it in anger and lay awake that night feeling wretched). The problem you have is when controlled light smacking doesn't work. 'Hit harder' feels wrong. The point of smacking is that it should be a shock to halt the behaviour in its tracks, NOT something that hurts.
I do think there are far meaner and neglectful things you can do to a child (than a light smack for appalling behaviour when there is no time for a naughty step), and some of these things are delivered by parents who wouldn't dream of smacking. I have seen people (good mums by some other standards) loudly dismiss one of their children as 'not very bright' in front of them, promise a treat and then casually withdraw it for no real reason, not bother to read to their child ever (or very rarely), ignore their child routinely when that child is trying to talk to their parent, 'pop out' to walk the dog and leave their child alone in the house etc etc. Not good friends of mine I may add, but I have seen it and some of those same parents would think me a terrible mother for administering a smack.
None of this is to say that smacking is great or not as bad as those things above. I do think though that MNs by definition will tend to be parents who care and therefore, we all have our views on what is good parenting and what is bad but generally, we probably all do our best and aren't cruel people or awful parents.
Reality is though that for us, smacking isn't working. And I'm glad as it is hardly a high point of any parent's relationship with their child.