Those that can, do. Those that can't, teach.
Usually, those people that are at the top of their field make very poor teachers. They simply can't relate to people that have to work so hard to do something that came so naturally to them.
On the other hand, those people that did OK in their field, but were never spectacular, usually make very good teachers. They are better at relating to their students since they had to study/practice their a$$es off too, and probably had to be taught how to overcome some of the same obstacles their students are facing.
I'd love to see Marino return to the Phins in some capacity...perhaps even QBs coach if thats what he wants.
But just because he was one of the best ever to play the game doesnt necessarily mean he would be any good teaching it.