For me the whole point of the Oasis is the shape, light weight and buttons. Reading it in the cover would defeat all those things for me.
I think the K3 was the last kindle I consistently read in a cover. I had a great light cover with a back strap for that one. With those perfect buttons on the edge, it was a dream.
The PW I never liked the cover or the PW really. I couldn't get it out of the cover so I had to read it in it.

. Had to get a basic then though as I hated the screen and lack of buttons on it. I had the PW1.
The basic was so nice and light, still is of course.
Voyage I did get a Origami and a 3rd party cover at first. But my wrists were getting worse so read out the cover, put in cover for storage.
I always intended to read the Oasis out of the cover.