It most cases children do not understand why the are being spanked. They do not associate the punishment with a specific behavior and do not learn how to change their behavior. There are a number of techniques that have been proven to be more effective and do not harm the child.
Developing and building a child's self-esteem is a critical task that takes a great deal of thought and education. Numerous academic studies have shown that spanking - indeed any type of physical violence - results in lower test scores and academic performance, a higher rate of aggression and a lower ability to develop a healthy self perception.
If the goal of spanking is to stop a specific behavior, it may work for a brief period of time simply by inciting fear in the child. In reality, spanking, regardless of frequency or intensity, is a violent act and the longer term effects have been shown to reinforce the unwanted behavior. Further, spanking creates feelings of powerlessness and anger and these feelings can increase over time, actually creating more of the child's unwanted behavior.
There are numerous alternatives to physically harming a child as a form of punishment, and these alternatives contribute to positive psychological, emotional and even intellectual growth over the course of a child's life. Cognitive approaches that involve conversation or "naturally occurring consequence" methods are far more effective and even create strong, trusting bonds between parent and child.
Yes, I think spanking a child as a punishment does more harm than good. Spanking simply teaches fear, and it does not teach children how to think critically or to learn from their mistakes. It is much better to use alternative discipline methods rather than corporal punishment. Timeouts, allowing children to suffer natural consequences and then talking about it and other approaches work far better.
In the grand scheme of things, spanking is pretty low on the list of actions that cause harm to a child and its effects are arguably negligible, but when used excessively or as a primary form of punishment, it can do more harm than good. It can cause a child to not only fear their parents, but also to believe that violence, not reasoned rhetoric, is the way to solve problems. We need to teach children to vocalize their feelings appropriately, not manifest them through physical violence.