@EdRick - I do not disagree with you that research and teaching are very different skills, and that people who are skilled at only one or the other can still have a place at a major university.
But what about smaller universities? They need people who are reasonably skilled at both. And keep in mind that a professor who does not research may not be totally up-to-date in his or her field. So such a person might be very well suited to teach intro level classes, but upper level undergraduate courses should be taught by professors who can teach, but who do at least some research.
On the other hand, high-level graduate students have reached the level where they are primarily self-teaching. Their advisors need not be gifted educators as long as they are cooperative and helpful with the student's own research process.