The key is, I think, knowledge, rather than experience. To use school as an example, one would generally hope that the person teaching A-level physics has a degree in the subject and therefore sufficent depth of knowledge to answer the odd penetrating and/or difficult question beyond the syllabus. The old system, where the FI course was based entirely on the PPL syllabus was fine when a huge number of instructors came from military backgrounds with the concomitant embedded knowledge and would pass on pearls of wisdom to new AFIs. It's a different ethos in the schools these days, so I'm afraid I'd be looking for at least CPL knowledge in any new instructor if I were still a CFI.