I'm not sure I fully agree with your take. Mentors are great, and everyone should have some. However, good clinicians help to improve both the band and the director. I always get the most out of a clinician visit during the hour I spend in my office with them after the rehearsal. When I have brought a clinician out for a few years, have seen all of their "tricks," and stop learning new things from them, I start looking for someone new to bring a new perspective. I approach clinicians with the attitude of "I should learn as much as the students do during this clinic." If I'm not getting something out of it, I look for someone new.