Back in the days when I was chunking hardware for bass I knew a guy in NW Alabamaistan who would rub his lure on the belly of the fish he or his buddy had caught to pick up scent from the fish. Something he had read somewhere. But I think it may be possible that flies that have caught fish would pickup a fish scent and become more productive. I am also a believer in the cripple look (which I tye naturally) and the predator nature of fish.
Again back in my bass fishing days, a friend would make 10 cast and change lures if he did not get a strike or catch a fish. Of course with a baitcaster or spinning outfit you are covering a lot more water than with a flyrod.