I like the law. However, it won't change a thing because those who currently smack will continue to do so, and those who don't will continue not smacking.
I smacked DS1 once when he was tiny. He ran away down the drive as we were leaving the house and into oncoming traffic. I caught him by the scruff of his neck as he was stepping into the road and, without even thinking, smacked his bottom. He was tiny. So tiny he still used reins some days. And I realised there and then that I'd lost control of myself and that I would never do it again. 12 years on I've not smacked again. If I feel myself getting frustrated I leave the room and switch with DH. I explain, I remind and I give consequences. But since then I never have - and never will - smack. It served no purpose that day but to scare DS1 into doing what I wanted and as a child who grew up in foster care, watching a parent lose control is a terrifying thing I don't want my own DCs to witness.
They're great, well-behaved and well-mannered DCs. Not smacking hasn't given them free rein to do whatever they please. They've simply worked out that doing the right thing and making good choices elicits better responses and feels better than making poor choices.