That's a tautology. "The type of spanking that does no harm does no harm".
OK, and since the parameters for this kind of spanking are so narrow and "no more harm than not spanking" is such faint praise, how about we just don't bother spanking kids at all?
You disregard the fact that there are groups of parents who find conditional spanking to be the most conducive form of discipline, given a particular situation. It's not the first resort - it's the last resort, when other forms of discipline have failed.
It's not administered angrily or harshly.
Some of the minds behind the research that are relied upon for these debates, admit that their studies have shortcomings. This does not mean that their studies are irrelevant by any means, as they paint a very clear picture as to the negate impacts that corporal punishment can yield.
There is a significant distinction, however, between conditional spanking and corporal punishment that is implemented in a severe manner without incorporation of other disciplinary tactics.
Source:
The case against spanking
The best form of discipline would be that which doesn't involve corporal punishment. However, I'm not of the opinion that a parent who has better success with conditional spanking, should feel guilted by society to change what works best in their household.
I'm inclined to trust my own judgement as a parent, knowing that I have not and will not participate in the spectrum of severe and abusive corporal punishment.