I've just started instructing in the last 6 months.
Finally, I don't have to pay 100 quid a pop, in fact I even get paid (if you can call it that

).
I really enjoy it and try to provide the type of instruction that I would expect to get as a new student starting out in aviation.
But the real purpose is to build time, as this is what the airlines seem to want (hopefully).
I do agree in part about the relevance of 100’s of hours Single Engine, out to the Practice Area/In the circuit to airline operations.
What I do think I have gained that may be relevant is:
1. Much greater capacity i.e. teaching, flying, listening to ATC, lookout, entertaining all at the same time.
2. Hand flying has improved massively.
3. Monitoring student/aircraft performance.
4. Constantly meeting and dealing with a multitude of personalities.
5. More experience handling the pressures to continue with a flight (usually personal income) that could compromise safety i.e. bad weather, overweight passengers, dodgy aircraft. etc.
FIS.