Steven, That is sort of my argument. All studies (that I am aware of) of what a trout sees in the water agree that they first sight wing tips of an insect or a fly coming into their "window" of vision. Then, they see the impression in the meniscus on the water of the body/legs; finally, when the object is close to them, the wings align with the body and show the entire insect/fly.
So, if fish are keying on mayflies I believe some sort of upwind is very important. When caddis are about, or were recently about, they don't need upright wings to be present.
As to the Harrop Hairwing Dun, most folks suggest it is sort of an all-purpose pattern, but not the best when fishing to picky feeding fish. When fish are actively feeding on a particular mayfly pattern, I don't think the Hairwing Dun would be the best choice and I suggests that Rene would agree. His most promoted/used patterns are more lifelike........the cripples and the more standard type upwind flies he and his family tie.