e-ink devices are indeed very fragile. Reading without a cover is waiting for an accident to happen. Even when the screen itself is not physically broken, the display can be. I know about it because it happened to me twice, the first time we had to replace the device and since I dropped it (I was reading on my couch, my baby daughter called, I turned around and the reader literally slipped to the floor), I had to repay it fully. From that day on, I never never never left a reader without a case.
And yet...the second time, it was never dropped, always carried it carefully in its hard sleeve in my handbag, and yet, one day, the display showed only horizontal black lines. Thank god that time I was obviously not the cause, and the device was still under guarantee so it was replaced. But it must have had a slight shock of some sort, I just don't know how nor when.
I'd advise a book-cover type, so you can read with the reader in the cover. Just my 2ct, but you know the saying, better safe than sorry....