My opinion is that for nymphing a highly visible butt section of the leader can help as an "indicator," but for dry fly fishing leaders should be "invisible." When dry fly fishing, the point of reference for a strike is the fly and not the leader. When one needs to "see" the leader, for example when fishing fishing flies IN the film, the greased leader technique is the method of choice.

While a high viz leader may not put off all the fish, I think it spooks some fish especially when you happen to cast over a fish or if it floats over a fish. If you can see it better for your backcast, the fish can see it better in the air. The sad truth is that you will never know about the fish that you spook.

Would a turkey hunter use a fluorescent shotgun or deer hunter a day glow rifle? Why use a high viz leader which will be part of your tackle that is closest to your quarry other than the fly itself?

To those that say that the color of a leader floating over a fish does not matter because the leader is a "shadow," I reply a leader is translucent and the fish see the color quite well.

Read The Angler as Predator or listen to Gary Borger's recent podcast http://www.askaboutflyfishing.com/