Of all the fishing books I own, the one I am least likely to part with is George Harvey: Memories, Patterns and Tactics, which was organized and published by Dan Shields.
Of the many fascinating comments Harvey makes in that book, a few regarding hackle and wings are relevant here. He was perfectly willing to clip the hackle from underneath a nice, traditional winged dry fly in order to get fish to take. He mentions how selfish (I looked it up, his word) he was about this and didn't tell his fishing buddies about it even though they were struggling to catch fish. Of course, no one on this forum would ever do THAT!
He looked upon the hackle as spent wings, not legs.
He experimented with pulling the wings off of a traditional dry fly and claimed the ones without wings caught just as many fish as those with. He nevertheless liked to put wings on.
I look at the duns of the blue quill and bwo (caddis are a whole different ball game) hackled dry without "wings" as a cross between what Harvey describes and what afishinado mentions. I think if the fish is looking for spinner wings, it works for that, but if it wants upright wings, it works for that too, and like Harvey I usually clip the hackle underneath. On larger flies and on light colored flies, the jury isn't in for me regarding imitating duns this way, even though the example that Harvey gives was in regard to sulphurs.
Harvey plays down parachute hackle because he thinks that silhouette is inadequate to imitate spinner wings. However, I suspect if he had fished them more, he would have caught just fine because, as we all know, presentation trumps pattern, and the man was a master at that. I know I like parachutes and use them often. Comparaduns, too, in # 14 and #12 especially.
There is one aspect of traditional wings that I ponder occasionally. You know how a winged fly can lean over to one side and then right itself, perhaps especially if you are casting sidearm (or you didn't tie the wings right), which in real world situations can be fairly often. This righting of the fly may mimmick a natural that is collecting itself a bit before takeoff, adding the desideratum of drag-free movement to an otherwise fairly static fly. I used to assume this "righting" of attitude was a turnoff, but maybe it can be a trigger just as well. Someone has probably written about this somewhere, but I can't remember it if it has.