- Backlit displays give you eye strain and make reading less pleasant. This can be solved with eInk but then you lose colour diagrams/graphs and even the largest eInk device is smaller than a single textbook page (and the layout within textbooks is key to understanding in some cases).
- Saving multiple pages is very painful. Almost all ebook readers (both apps and devices) allow you to save your current page, close the "book," and then continue later. Very few allow you to tag multiple pages with "sticky notes" and then jump back to any of them.
- Some people like to highlight their $100 textbooks or to write notes on the pages. I am not one of these people, however that is a legitimate argument against buying the digital book. You rarely can do this type of thing.
Unfortunately eInk was here to save us, and I think as a technology it hasn't really succeeded. It gained popularity for a brief period, but then the iPad got popular and people just decided to read on the device they already own (and then give up, and buy paper copies because it was so unpleasant).
So unless someone invents an RGB eInk device, I think we'll be stuck with either paper books or terrible backlit flicky displays forever. Too bad if you ask me.