I think the reason why it's banned (like it is in Norway) is to send a message to people to be extremely careful with physical punishment, just like speed limits are set to tell people to slow down. Nobody goes to jail or gets a fine for smacking the hand of their child in certain situations, but it clearly states that it's undesirable to get physical if it can possibly be avoided. A ban also makes it easier to get parents who really hurt their children easier. They can't hide a real beating behind "it was only some good ol' spanking", because good ol' spanking isn't legal either.
If that's true then I don't think that the intended effect is necessarily what always is achieved. For example, even though it's not gone quite as far as an outright ban in the UK, there are so many parents who have children who have temperaments that are just beyond being reasoned with - or perhaps the kids are just smarter than their parents and run rings around them all the time - but whatever the case, these parents feel at a loss as to how to discipline their kids, and are petrified of having their kids taken away from them if they use corporal punishment at all. And if it's not having the kids taken away from them that they fear, then the sheer stigma of having it known that they spank their kids is enough to deter them.
I've had many very judgemental and disapproving looks for the odd clips around the ears I've given my kids, even though the kids were so clearly unharmed that they were actually laughing!! I've even had parents of my kids' friends coming to 'have a word' with me because when our kids were playing together, it's been mentioned that one of them got a spanking for something, so I've had people playing vigilante on me and stuff, even disallowing their kids from coming to my house to play with my kids for fear that I'm some violent asshole that will spank their kids as well ffs.
So the effect is that they simply don't get disciplined, and so behave with very little respect for anything or anyone. Any 'official' system that's made to deal with them also avoids physical punishment, and they just learn how to play the system to get what they want out of it. I carry on disciplining my kids now as I said before, without the use of physical punishment but I think it's only possible because I established a strong foundation when they were much younger, which I don't think I could've done as effectively without the use of spanking. And back then, though I carried on as I saw fit regardless of the stigma, I always felt that I was gambling, y'know, running the gauntlet and any minute now I might have some self-righteous social worker come to call because they'd been tipped off by some concerned citizen...
I've also seen at the child psychiatrist clinic where I go with my little Aspie, posters all over the wall defining child abuse and detailing its effects and the penalties from the law. It includes 'shouting' in a list of things that constitute child abuse... so parents seeing these posters and living in this social climate are basically being told that they're not supposed to hit their kids, or shout at them or say anything negative to them - if you're not a very clever and creative person, as most people are not, then I fail to see what options they're going to feel are left to them. Which, IMO, is why they're all there with their kids because of 'behavioural problems'.