I don't like smacking... I think it's ineffective BUT I really dislike the idea of criminalising parents.
Consider these scenarios:
a) a stressed out mother of a toddler gives a light tap on the hand or bottom after said child has been hitting/pinching her mother and all else has failed. This is the only time smacking has been resorted to Unfortunately, there is a witness and she is taken to court. What you going to do? Take the child away from her? Is this REALLY in their best interests?!
b) a 3-year-old, despite never being smacked, joyfully goes around telling all and sundry that his dad smacked him (yes, my ds1 did this occasionally - would go through a phase of saying "you hit me" when we'd done no such thing). There is no physical sign of violence, but because a nursery worker has overheard it, action "has" to be taken.
Legislation is NOT the way to go here. FFS, social services are already overloaded and can't manage the cases that clearly already fall outside the law! EDUCATION is the way to go. Have compulsory parenting classes when people fall pregnant if you must. Don't make an even greater divide of US and THEM against the State.
My dad was slightly over the "reasonable chastisement" stakes. No, I didn't like it. Yes, I am very aware of not going that route with my children. Do I think I should have been taken away from my parents at the time? Hell no, it would have destroyed my life.