The only things I can think of are potentially better PDF viewing, and simple games like chess or solitaire (I think Koreader already has those however). Seems like a lot of effort to go to for little benefit.
Text-based use is actually excellent.
The general principles of eink UI/UX design that I've been formulating are:
- Persistence is free. If the display doesn't need to change, it draws no power, and will display the last-painted content indefinitely.
- Pixels are cheap. The pixel density is high, typically 200--300 dpi. This is better than all but the best Retina displays at the low end, and parallels paper at the high end.
- Ambient-light and bright-lit environment performance is excellent, most especially direct sunlight. Frontlight or backlight systems make low-light performance better. Principle mode is reflective rather than emissive display. Shading and pigmentation is subtractive rather than additive.
- Paints are expensive. They draw power, are typically far slower than emissive displays (1-8 Hz rather than 60--200+ Hz), and result in significant degredation (ghosting) at higher modes. Repainting the screen is highly distracting.
- Contrast is limited. Modern displays are typically 16-shade greyscale. Line art and high-contrast block regions display well. Halftoned images appear better than direct rendering. Shading and any deviation from pure-black-on-pure-white contrasts degrades performance.
- Colour is nonexistent in most cases, and limited where present. There are colour e-ink displays, though these tend to be muted and pastel, best suited to comic books rather than high-saturation photorealistic presentation.
- Paginated navigation is highly preferred to scrolling. Pagination is predictable and executes a single repaint for a navigation action. Scrolling at high-quality display levels creates noise and takes time, at lower-quality display levels remains difficult to track accurately and creates a large level of artifacts on display.
- Dark-on-light is preferred to light-on-dark, generally. Night-mode themes typically don't render well.
In practice you'll see full screen refreshes be the high quality type, and smaller refreshes being of the fast type. That said plenty of e-ink displays don't bother with the fast refresh or even with partial refreshes and just always do a full screen high quality refresh. For E-books that's probably fine.
For you, I guess? Lots of people are just fine with it, including myself, especially against the background of the great reading quality.