You've stated yourself that a consequence could be a number of things, spanking could be one of those "things". Sometimes even the hint/suggestion of spanking, even to a child who has never been spanked, is enough for the child to realize that if their behavior maintains... they will be punished.
A combination of many different methods of discipline should be used to mold behavior. At the same time, a combination of many different types of consequences should be used if their behavior becomes unacceptable in the parent's eyes. Spanking, used properly and not as an emotional outlet, is just ONE of those types. It should never be used exclusively, just as time-outs should never be used exclusively... THAT's when it becomes ineffective.