I will wade into the shark infested waters here, LOL. I am a father of eight, and the oldest of seven children. My siblings and I were spanked, and I have, on occasion, spanked my own children. I will not launch a defense of the practice as I tend to agree it is contrary to the idea of the NAP.
That being said, I would say this, it is hard to argue with results. Out my six siblings, two adult and two teenage children, not one single arrest, major psychological disorder, bout with depression, suspicion of neglect or abuse. That is not to say, that had other means been used, the same results might have been achieved. With my own children, here has been the criteria, and I will say that instances of spanking are very rare:

1. A child is in direct defiance of a rule that puts others at risk, knows and can articulate that they understand the rule and chose to break it of their own volition and shows no remorse.
2. A child has willfully and intentionally injured another and shows no remorse, and has expressed that they were not acting in defense of their person, or the circumstances have made it clear that this was not the case.
   At times when I was spanked as a child, I felt it was done out of frustration. At no point did I ever feel my parents were out of control, or acting in disregard for my safety, nor intending to inflict lasting harm on my person, nor did they. I have worked very hard to eliminate emotion from the process. The punishment has always proscribed and described to the child, previous to the offense, and has only been administered as a known consequence for breaking a known rule and once administered, the correction is considered finished. (of course there is still talking and explanation, etc, but punishment is ended)
   I do believe that in large part this method of discipline has been used in my own life based on my own experience with it as a child. Here is my advice to parents: choose a method for training your children that takes into consideration the fact that you are raising them to be an adult who can live with others, in harmony. Make sure that your rules are clear and whatever discipline you use is applied only when known rules, or logical extensions of those rules are broken (ie: the child does not need a complete list of property that belongs to others that cannot be taken without asking to understand the concept of stealing)
   There were not many rules in my house growing up, but those that existed were enforce consistently, giving us a clear option for avoiding conflict, by following the rules. Anything outside the lines was permissible, until it became an issue, at which time a clear understanding the new rule was put in place. I think the two greatest harms that parents can do is to 1: discipline a child for being a child (doing something they consider wrong that the child simply did not understand) and 2: not disciplining a child for direct, intentional misbehavior.
   I will write a book on parenting some day, the first line of which will be, Our supreme duty as parents in this life is to raise our children in a manner that ensures, as much as is within our power, that they do not turn out to be assholes.