Spanking is an essential tool (among others) to raising good kids, and if done right is hardly ever needed after the age of about 6. Oh, and by the way my kids (4 of them) love me to death and take every opportunity to love on me, kiss and cuddle with me.
I get unsolicited comments from total strangers all the time on how well behaved my children are. A child properly disciplined when young will hardly ever need to be spanked beyond the age of about 6-7 (they threat alone will suffice).
I have had several relatives with older children try to get me to take in their teenage delinquents and I wanted nothing to do with that idea. It's too late by then! If they don't understand right and wrong, discipline and punishment by about 7-8 your screwed.
I'm not even sure it has much to do with moral reasoning. Humans, like all animals, are creatures of habit. The point of spanking is to prevent children from forming bad habits, of thought and behavior, that they will take with them into adulthood - habits that will impair their future success or even their ability to function in society.