I think a teaching pro needs to be able to TEACH tennis, not play tennis.
Makes no difference what level the teaching pro is, what counts is how well he teaches the fundamentals to beginner players, up to above his own level.
A teacher is a teacher. That is the job, not demonstrating the difference between Roddick's serve compared to Federer's.
And a teaching pro doesn't necessarily need to coach potential WTA and ATP pros.