Isn't it a question of degree though? Those that are consistently exposed to physical abuse end up with more problems that the "slap" seeks to cure.
I know the threat of a spanking was used from time to time when we were kids. But to be honest, the threat of "you wait til your father gets home" was worse (I don't remember him ever hitting us). I am sure we had the odd spanking from mum, but I don't remember it, I tend to the view that it was a reasonably rare occurrence. As a result I like to think that my siblings and I grew up as fairly well balanced useful individuals (apart perhaps from my involvement in this place)!!!
I DO however remembering threatening to call the NSPCC and reporting mum for child cruelty when my mother banished me and my sister to our rooms one day following a particularly nasty fight between us. So she looked the number up, handed me the phone and told me to go ahead. Now THAT worked!!! lol
I once intervened in the street when a woman was screaming at her hysterical toddler that she was going to "pull down your fekkin pants and tan your fekkin arse and fekkin give you something to fekkin cry about". THAT sort of behaviour in my view is more of a threat to society than sparing the rod.
Hence children grow up thinking that violence is the way to end all evils and things they disagree with.