I'm with @egregorian on this on this one. Kids don't learn to be decent, responsible citizens through spanking -- they learn to avoid abuse. Spanking stunts their verbal reasoning skills (this from recent research). On the parental side, once you've given yourself permission to discipline this way, you can too easily do it arbitrarily after a bad day at work or when your child invariably bruises your ego.

As for the "spare the rod" bit, the rod and staff were shepherd's implements. The staff was the offensive weapon to be used against wolves and other predators. The rod was not used to beat misbehaving sheep, but to give them guidance, keep them with the flock, etc.