I like book covers, especially when in social spaces. I like being able to see what other people are reading, without being intrusive, on the Underground or in coffee shops. It's a quick, superficial survey of a snippet of current reading tastes and trends. Sometimes, seeing someone reading a favorite book or author, that glimpse makes me happy, an invisible commonality with a stranger.
There's no point making a book cover visible on an e-book reader to other people. I can imagine several ways it would be done. A protective gel skin printed with the image and stuck on the other side - but then it's so much fuss to bother changing when moving on to the next book, so why bother? The image and the book need not correspond, but that discrepency partially defeats the point of having it there in the first place. (It would be more akin to displaying pins on bags and logos on clothing.)
The image and book need not correspond for a two-sided, two-screened e-book either, theoretically designed to display an image of the cover to the world. In practice, such an outward-facing screen would be widely use for other purposes instead, whether public service weather reports or insults.
By making reading even more private an experience than it already is, the e-book pushes its brand to the forefront as the visible point of interest, not the content. This allows much greater privacy, but it strips out most of the interesting information shared in the physical presence of a reader.