I do not think that spanking is a good way to discipline because it is to easy to let your own anger and frustration enter into it. To put spanking another way - you would be saying to the child, I am going to cause you physical pain to get you to behave in a certain way. We are adults, they are children so why can't we use our brains and our maturity to figure out how to get a child to behave without hitting.
We raised two kids without spanking and they are lovely, productive adults. There are so many ways to discipline a child without hurting them, but it does take more time and patience then just hauling off and whacking them.
I also think of how the child feels. Children have no control in their lives. We rule everything for them. To hit a child as punishment can be terrifying for a child and make them feel out of control and trapped. I just don't like the idea at all.
(I can see the little red disapprovals arrow fly now!) People tend to vote things down if they disagree rather than if it is a part of the discussion as asked in the question.