Those that do have kids and have not found the need to spank them AND who can with a clear conscience say that they didn't turn out to be hellions should be thanking God (like Irish Melkite did above). When God gives you docile and obedient children who do not need to be disciplined more fiercely than with a strong word it is a gracious gift and not something that makes you in any way morally superior to those who have not been blessed with this boon.
I certainly feel no moral superiority over much of anyone. God has blessed me with children who exhibit all of the good qualities that I cited above - but He certainly didn't include in that blessing anything approaching docility on their part. Personality-wise, they span a spectrum but not a one of them is without a healthy dose of outspokenness that manifests itself both as self-esteem and as a willingness to stand up and be counted in defense of anyone they perceive as an underdog. Their Irish and Scots heritage is manifested in more than their red and blond locks; their tempers and dogged stubbornness mirror those traits in their Dad and don't always serve their own best interests. But, they're fiercely bound to the notion that, at the end of the day, one must never walk away with anger in one's heart or leave anyone with angry words being the last thing they heard.
As far as the harm that spanking inflicts on anyone, I am firmly of the opinion that some, who would never allow themselves to physically strike a child, do incredibly more harm to their children with words - words that they consider to be emblematic of loving discipline but which actually cut to the quick and devastate the one on whom they're inflicted. I know people whom I wish would physically discipline their children because I'm certain that it would be administered much more gently than the verbal thrashings which are standard fare for all manner of transgressions.
But, in either instance, the ultimate problem lies in timing. As some have mentioned, one should never punish a child in the heat of anger, as therein lies the danger of being truly abusive. But, just as is the case with animals, punishment that lacks immediacy is inherently ineffective. When time - a cooling off period, if you will - intervenes between the events that merit punishment and its administration, there is a mental disconnect between the two in the child's mind, the point is lost, and it comes to be perceived as just outright meanness on the punisher's part.
That, to me, is the strongest argument for alternative, creative, disciplinary methods to be employed - in the immediate aftermath of the circumstances that merited them. "You did this and there is a consequence, which you must face - now."