Or to turn it around, why would you pay for an instructor to take you diving in an enviroment you dive routinely and is very familiar with and some divers even know better than the instructor?
I find much great input from diving with a variety of people, however I often learn as much from diving with less experience people as I do from instructors. Not because the instructors is bad, but because their less likely to make mistakes I can learn from. Of course I can (and have) make mistakes to learn from myself but Im not gonna do them on purpose either..