Well, if you're far too liberal, then they grow up feral. Hitting children should only be a last resort (again, by "hit" I mean like a spank), if all other vocal warnings and other attempts fail, there is one that will no doubt succeed: pain.
You touch fire <pain> you refrain from touching it ever again.
You constantly bump your head into objects when running at home <pain>, you learn to walk at home.
You disobey your parents verbal warnings, so they hit you <pain> you learn to listen to them.
The excellent thing about spanking is that, it acts as a substitue for the actual consequences of a child's actions that could've been much worse - or even fatal. By establishing a system where if the child disobeys, the get slapped, is like using money to buy goods, rather than trading your own goods for others - it's a "currency" for learning lessons and instilling discipline.
Children could totally disobey their parents when they tell them to look when crossing roads, or to not play with fire, and their actual punishment would be one Hell of a lot more severe than a simple spank (hit by a car, burnt all over the body), and both are fatal. Hypothetically, providing that you establish an understanding in children's heads not to disobey adults/parents, then the "code" of discipline can be extended and used for many other "lessons" that kids can learn.
For example, the lesson not to run with scissors, and not to hold them blade out. Now, if the said child is already used to being spanked if dosibeying their parents/guardian, then I'd imagine they're more likely to listen to them, rather than waiting to find out the actual punishment of running with scissors blades out (i.e stabbing themselves or stabbing someone else).
If the child has been brought up by parents who don't spank or don't discipline, then it's more of like a "let them do whatever they want, untill Nature kicks their arse" attitude, and the child would probably run with the scissors in hand, and stab themselves/someone.
My point is, that spanking is like a "currency" of discipline, that can be used to provide a physical punishment (and therefore lesson) of pain, without having to make the child endure the actual punishment of their actions.
For example, it's much better to slap your child for attempting to cross the road without looking, rather than just letting them run accross without looking and "learn" by being crushed by a vehicle.
Not only that, but you'll instill a sense of discipline, and the kids will then only need a verbal warning, and won't be slapped, because they'll know that if they disobey they will get slapped.
In my eyes, you've gotta keep a balance between nurture and nature, nature is far too heavy-handed and will just get them killed, so spanking and a sense of discipline to me is merely just Tough Love.
However, I only support this idea if it is executed properly, and far too many people just slap out of aggression and just abuse their children.