Sandy Banks is correct that the scientific studies are just correlational and don't actually prove that spanking is harmful. But she omits important aspects of the debate.
Spanking actually works as a means of suppressing behavior. Physical punishment is still the most powerful method for immediately suppressing undesirable behavior, which is why police and soldiers rely on it. With certain extreme behaviors — playing with matches or biting other children — spanking might still be the right intervention for some children.
But the real problem is that most parents simply don't know about other, gentler ways of suppressing behavior, of which there are about 50. Instead of engaging in an endless debate about spanking, our main effort should be in educating parents about alternatives.