Over the past years of my education in Ghana and the United States, I have come to recognize the vast distinction in teaching-learning outcomes between a professor without the teaching credential/background and a professor with a teaching credential and background. As a teacher myself, I can relate specifically to this question.
Teaching is a art and also a science of communicating and transmitting knowledge in a clearly define and measurable manner. A professor without teaching background may be excellent at the subject matter (content), but theoretically lacks the potency to effectively communicate knowledge building on the art and science of teaching. A professor who has a teaching background doesn't only teach content, but also process and is better equip than his counterpart; that is, those without previous teaching background. What do you think? I am not suggesting that someone without a teaching background cannot be a better professor. But someone who is a teacher and becomes a professor is better off than the other. Intuitively, not all professors with a teaching background are better professors either. It depends on the individual for the most part, but if two professors teach the same subject matter and either one of them fits into one of the categories describe above, one will quickly recognize who is just the "professor" and who is professor-teacher.