Noise cancelling isn't hearing protection, fwiw. Ear protectors do a much better job of that, too.
Also when you're trying to sleep on a plane, Bose's active noise-cancellation stops working when you lean your head hard against a headrest (such that the earmuffs touch the headrest). I think it's just the nature of active noise-cancellation. I've resorted to earplugs when trying to sleep while flying.