Skydriller,
What if you owned the aircraft and needed a bienniel type of review because the regs say you do, and you wanted to do it in your own plane. Let's also assume you had more hours than the instructor.
You need the instructor because you need his signature, and you might also learn something (as might the instructor). Who would you want the PIC to be? Who would be in charge of your plane if you had an engine failure?