I think any form of physical punishment for children should be outlawed. I've seen both extremes and its not a pretty sight. I know one guy in prison for killing his father due to the physical and mental abuse he suffered as a child. I know a woman who is a train wreck due to her parents trying to befriend her and withhold all punishment. IMHO raising your child can be done without ever laying a finger on them. It is more difficult especially if your child is willful but the results are outstanding. I relate raising children to training dogs. The same principles apply. You start young. You give them lots of opportunities to succeed. You give them as many yes's as possible. You limit the no's to whats absolutely necessary. You over celebrate their victories and use their losses as learning opportunities. Your disapproval is more than enough to punish a child if you have done everything else at least halfway right.
Asclepias, there are two things I want you to know before we discuss this further. 1), I respect you. 2), you're OK in my book. That said, we disagree a bit on some stuff.
Physical punishment should be legal, so long as it doesn't cross any boundaries. Using a paddle or a hand on the rump is very different then slapping or punching the face, or beating, kicking punching, etc. Evolution made it so that the butt has fat on it, to make sitting more comfortable, while also making disciplining your children easier for both of you.

Sometimes young kids are little demons. I was one of them. Reasoning didn't work. You couldn't reason with me, therefore, you
couldn't stop me. Little ones with Attention-Deficit Hyperactivity Disorder can be so hyper and inattentive that only a few things will drive the message into the brain, and ensure it's remembered. And along the lines of being a hyper, evil, and intelligent brat, I soon learned that buying my love and obedience could be abused by me. Hitler did it, too. Sending me to my room did nothing, because video games. Having me stand in the corner did nothing but give me time to collect and organize my evil plans. Every little chink in my parents' armor was learned and abused, making it easy to come out ahead in bargaining.
Then the day came when it all stopped. I learned fear. After breaking something else Mom and Dad brought me into the living room, and I was notified that my butt was going to be spanked. This was new to me, because everything had been going so well. The very thought of this impending attack on my behind made me think that maybe, just maybe, my parents wouldn't lay down and take it any more. So with the bending of my body over Dad's knee, and the torture of the eternal two minutes, fear was not only learned... but remembered. Needless to say, so did respect. And with that my dreams of global domination came to an end.
If you my friend can get your children to obey you without having to spank their behinds, you are fortunate. That is rare, and was certainly not the case for my parents. Interestingly enough, we trained our puppy by scruffing her as punishment for biting people, running away, etc. She was a very loving, obedient, loyal sweetheart, and never got in trouble with the pound.
What you've mentioned about celebrating their victories and using their losses as learning opportunities is good, and I could see using that side-by-side with spanking out really bad behavior (maybe a caveat here would be to not over-celebrate, though?). If you were my parent, and I was very young, you would have to control me with spanking (give me very good reason to fear, respect, and obey you), or I'd likely burn your house down because no one's given me reason to not dare explore with matches.
Sometimes spanking is necessary and justified, and if you have reason to do so as a parent, you also have to keep it in moderation.