To spank or not to spank - that's a question that plagues many parents today. Those who don't spank have a valid point. Hitting teaches hitting, and they don't want their kids to grow up thinking problems can be solved by using physical violence. I guess that describes me too. I do believe spanking can be appropriate for toddlers in dangerous situations, when you need to get their attention and make your point fast.
Some parents also spank for willful disobedience, or when a child is inflicting injury on another.
The main thing, I think, is to be wary of when spanking is a vent for your own anger or impatience. Discipline of any sort should be a learning situation, not a vindictive one. The hand, some say, is the only tool. It's quick and it's personal. Others say never use the hand, use something more neutral, such as a paddle or a wooden spoon.