I have rarely used them on other devices, and not at all on any recent devices including my last 2 phones (both of which had Corning Gorilla glass) and my last tablet. All of these had quality cases, and my usage was such that I didn't have big issues with screen damage.

But now my usage has changed a bit, and a few in this thread have alluded to similar usage. Besides my other devices, I recently upgraded for readers and now own a Kobo Forma, and a Kindle PW4. The way I like to use these is without a case to keep the weight down. Given that, there's probably increased risk of scratching the Kindle when I shove it in a pocket (since it is going to be my 'portable' reader, while the Forma will stay home). I'm going to try one of the well rated (ArmorSuit) matte screen protectors and see if, like @HarryT mentioned, I run into any kind of issue with reduced visibility.