Sleeves are a good protection against dust, and I don't have to put up with added bulk when reading.
I used covers when I had 6'' readers (up until the Oasis 2); since then I've used only 7'' and 8'' devices and I've found cases make them too uncomfortable to hold. I only ever hold my ereader in my hand when reading, as I'm nearsighted. I also have 3 cats, so keeping a fragile eink device somewhere else than in my hand while I read is pretty dangerous.
