Harmful in the sense of raising children who have never had any real boundaries growing up and so don't respect a parent's authority, are impulse-driven, are unable to delay gratification, have no work ethic, etc. These traits are detrimental to academic careers and job prospects.

That isn't to say that all children who aren't spanked grow up into these sorts of adults. But some children inherently have more behavioral issues, or are brought up in bad environments with a lot of negative influences, and they can grow into serious deviant adult behavior if not properly addressed when they're young. Corporal punishment is an effective tool and it needs to be on the table unless proven otherwise.