Will keeping the iPhone plugged in most of the time be bad for the battery?

Since the iPhone's battery is rated for a limited number of full charges, I was thinking that I'd just leave it plugged in most of the time so that the phone runs off of external power rather than draining the battery. This should reduce the number of times that the battery needs to be charged up, thus extending the overall life of the battery. Does this reasoning make sense or would keeping the phone plugged in actually cause more harm to the battery?