To find out and to be fair to both of us, perhaps you would specify the particular arguments you believe are unfounded or lack scientific support. It appears to be a risky practice for which any temporary benefits are inadequate justification. Spanking is no more effective in the short term than several alternative methods which do not hurt . It has never demonstrated any measurable, long term benefit. That is a matter of opinion and legal sanction.
Physical aggression is physical aggression.
People have decided that some types of physical aggression are illegal and some are legal. They have decided that employees, prison inmates, soldiers or sailors, and, more recently, spouses may not be hit. Spanking children, though, is a very common, physically harsh and potentially injurious treatment which remains legal. The differences between frank child abuse and common spanking may or may not be frequency or intensity, but one difference is certainly legal. But I think that frank physical child abuse and spanking are compared better when we consider the similarities rather than the differences.