If you're only reading at home and the reader gets moved between a chair and your bed and nighstand for instance, it's fine without a case or cover.*
If you take it out of the house though, which means in pockets, purses, backpacks, where it's easier to even slightly flex or have some other object pressing against it even a bit, you need a good case/cover.
Whenever a reader is new, it never leaves the house until the cover has arrived also.
*That's provided you don't have cats jumping on it or kids pounding on it or married to a clutz who will simply make it fly off the nightstand.