I'll elaborate a little on Blake's "never found them useful" comment. Part from him and part from my own observations.
A stingray, stratus, glide will all turn right when thrown rhbh. They are all understable.
I believe part of Blake's issue with that is if you throw an understable disc, you are typically looking for a shot to go right. With that in mind, if you throw an understable disc wrong, it will dive into the ground. You are following my (now old) axiom of throwing flat and letting the disc do what it's supposed to do.
Blake's preference, and one I'm going to work with this year, is force a disc to do what you want. That way, you know it's going to do it. With the right turn need. If you take a disc and force it to go right, you know:
a) It will go right and
b) It will probably come back to flat/stable at the end - depending on how stable of a disc you threw.
That said. Now, you have one disc that serves two purposes instead of two discs.
In addition, if you throw that one disc in several situations, you're more likely to learn all that disc can/will do for you and be more apt to throw it correctly (the way you need to) more often. With the single/special use discs, your usage may be so sparse that your percentages of throwing it how it has to be thrown go down.
My best example is one of our local courses from the shorts. You can use a glide on probably 5 of 18 holes. However, you can throw a putter on about 16 of 18. If I learn how to throw my putter on all those holes, I should increase my proficiency at a faster rate than if I choose to throw a glide on 5, a coyote on 6, etc.