First I've heard of the FOI...how do you obtain that?
Instructing is a logical time-builder, but it is sad how it is often just a stepping stone.
That wouldn't be too bad if every school were required to have a Head Flight Instructor who was a career instructor or who had come back to it.
I think regardless of the subject, teaching is the most important task there is. Doing is important, but good teaching of those who will do makes all the difference.
It's shameful how bad most teachers have it.
On a side note: looking back, I remember that some of my instructors didn't quite fit the stereotype: My first wanted only to fly island-hoppers in the Caribbean (hope he's there right now- what a nice guy!), and the one after him had to quit to go help run the family business. The rest were bucking for "the corner office"- left seat on an international line, flying the big iron. Some of the others at that school were so hungry for hours (and money), they'd steal other CFI's blocks in the schedule book: more than once I'd show up for a lesson, only to find that somebody had scratched out me and my instructor and inserted his own lesson. Cute. The worst was probably the oldest guy there... desperate, I guess.

My favorite instructor (who soloed me) had an interesting path: he was also a mechanic for the school (very comforting on my dual flights with him in some of those old rattlers!), and his career goal was to fly for missionary groups in the bush somewhere. He and I parted ways when they closed shop at N07, and I went back to KTEB to finish up. Not sure where he went to, but I'm certain he is either flying mercy missions above jungles full of rebel armies and poisonous snakes, or is well on his way... and loving every minute.
That's probably why he was my favorite- he had a very relaxed and thorough approach to exploring the edges of the envelope, and making every decision and action count for maximum efficiency and gain... and he just plain loved airplanes and flying.
He was also never easy on me but always calm, patient and focused thoroughly on flying the airplane.