A cover also protects against corner/edge drops. Even if the cover is open the edges usually protrude beyond the side of the device and will cushion the fall.
I suspect that the need for a cover versus sleeve depends on the type of reader that you are, and I'm probably more like JSWolf in this regard. I carry my book with me everywhere. I read on the couch, at the table, and in the [let's not go there]. When I go to my car, I take my reader and turn on Text-to-speech.
Anyway, this means I often carry my reader open as well as a stack of other stuff I'm taking with me from place to place. And I do have occasional drops when it is opened. If you only read sitting down on the couch then you won't have this issue.
I have this picture of me as this person who never puts down their book and juggles two or three different things at a time. My nose is buried in a book and life may sometimes pass me by. Sometimes a thing or two that I am juggling will dropss (literally)
. On the other hand, I have this picture of a 'sleeve' user as someone who is 'prim and proper'. They schedule reading time for an hour and go to pick up their reader from the shelf where it is stored away. They sit in their reading chair while maintaining correct posture and carefully remove their reader from its sleeve.
Or maybe you all read like me and its just that I'm a klutz
.