How about a research-assisting app, for text books? I wouldn't want the standard reading app to do what it would do, the extra UI elements would get in the way. And I wouldn't want to be stuck to a single one, there are multiple paradigms for different use-cases. So either clutter, or options-overload.

Similarly, from everything I've seen on E-ink devices, I could write a reader I'd be much happier with. For instance: half-page updates so one can speed-read without pausing between pages. Without the ability to do so, they're relying solely on their internal developers to make correct decisions for everyone.