My reading time with the Oasis without the cover is about 99%. I only read in the cover when I am closer to low battery on it, which doesn't happen very often. I always read without the cover and when I take a break, I stick it in. So its always topped off.
The shape and lightness of the Oasis is the reason I got it, so reading with cover would defeat that purpose. I much prefer how it feels in my hands naked.
Oasis naked that is.
Now I mostly read at home, so not sure how someone that takes it along a lot would deal with it. I have read in the doctors office for a while, taking out out while the cover was on my lap and then putting it in when my time came to be tortured.