Quoted from Slashdot:
This stuff is far from unusual; in any reasonably large department there's going to be someone who's too incompetent to ever get anywhere in the research side of things, so where do they wind up? Teaching undergraduate courses instead. The best minds cloister themselves in research and graduate classes; the worst minds get to teach the next generation.
Quite. An extension of "those that can't do, teach". Very familiar to me.