Many years ago an instructor passed on this piece of wisdom when I asked him about accumulation of hours and what people would look for in hiring pilots.
He said " It's more a case of how many experiences you can accumulate than how many hours. You can end up with 1,000 hours experience or you can have 1 hour's experience a thousand times." Perhaps that is the issue when discussing the value of instructing.
Hardly a new concept but very succinctly put I thought.
Cheers