Marinaro designed these flies for a more real-looking imitation to fool the wise trout on the limestone streams of Pennsylvania. However, these flies will perform well over any difficult trout, - especially at those times when the trout have become selective, choosing only one specific type of bug to eat when there are others on the water. These flies are also good in low & clear water or any place the trout has very good visibility & plenty of time to inspect your fly before it decides to eat it or not. Obviously, this style of fly can be tied to imitate any species of mayfly.
There is one down side to these dry flies: they are difficult to tie. This is not a pattern for the beginner tier. But with some practice, time, & patience anyone can learn to tie these flies.
To that end, check out this video below showing how to tie these thorax dries. So far, this is the best video I've found for this pattern. Videos for thorax flies are great, as it can be difficult to learn how to tie these even with the best of books.