Periodically I see threads come up which list skills that the OP considers redundant or useless, and invariably, Disengage is on that list.
However, since starting to do the Plaguelands instances at least, I've started using it, and a couple of times it's actually been a lifesaver.
The key is how I use it. If I use it as a standalone ability instead of Feign Death, it doesn't do much. Where it is effective though is if I'm fighting a mob in relatively close quarters, Feign resists, and then I immediately hit Disengage after that.
Aggro will get passed to my pet, and I can then jump back, use a macro to apply Serpent without starting Auto again so I give the pet time to rebuild aggro a bit, refresh Mend Pet if needed, and drink a healing pot to bring myself back from the edge.
So I don't think of it as a standalone skill; it works if used in a two-step sequence, after Feign Death if that resists.