Back in my CFI days I would get sort of irritated with the part time instructors that held other full time jobs who would be blocked off most of the week. In my opinion it was bad for their students who's time to learn may be limitied and on top of that would have to coordinate a lesson with the instructor's available time. And no, I wouldn't fly with those students unless their regular instructor asked me to because of vacation, etc. or there was a personality conflict and the student wanted to change.