Here are my thoughts on this, which generally aren't worth crap.
As an instructor new to a new school/company, I would think as part of the on-boarding process, it would make sense for the new instructor to attend as a student to audit the course, understand the curriculum, etc.
As an existing instructor, continuous training and improvement should be something that is pursued, and I value instructors that do this. There's always something to learn. In one of the past Magpul courses I took, there was an instructor from another school attending.