I suppose that's a silly question, especially since ebooks themselves really haven't evolved yet. They're still basically "print under glass", quick-and-dirty conversions from print, so it's no surprise that the covers are still as static as the print books they're based on.
The current book cover was designed for the physical world, of course. It serves as a billboard on the shelf for those rare instances when the book is face-out in a store. Covers are intended to be as unique and memorable as possible, selling the key features of the product. But they're also designed to be a lot larger than ebook covers ever appear on screen. So what happens when a print book cover that's 6 X 9" is reduced to the size of a postage stamp? Details are lost, the selling impact is diminished and the branding impression is compromised.
As I scroll through the various category pages on Amazon I can read most of the titles, but almost all of the subtitles are unreadable. A better solution would be to automatically pop up a larger image of the cover as I mouse over each one. Btw, do you want to see a publisher who understands the challenges of branding with smaller cover images? Look at all the Lonely Planet product icons in iTunes. You'll see they use the same template across the entire library and each title is distinguished from the others with three large letters: The location's airport code. Brilliant. Yes, I know that's a very unique vertical market. My point is to show that you can create something unique and memorable, even when the real estate available is as small as an icon.
There's plenty of innovation yet to happen with the content of the book and I hope the same proves true for the cover. Why do covers have to remain static? As I mentioned above, why not have them auto-magnify when I hover over them? Maybe have that pop-up show not only a larger cover image but also a few more details (e.g., short description, quick breakdown of 5-star vs. 1-star reviews, etc.)? How about making those cover images flippable? Touch it and the cover flips to show that info I want in the magnified version, just like the back of a baseball card? Then make sure I can drag that object off the page and drop it onto my own website, with my review and/or affiliate code built into the resulting link.
There's so much more we can do with covers but, again, we need to stop looking at them as simply static, digital renderings of the print cover.