Exactly. The basic reading experience is better on a Kindle, but once you use the glossary, take notes (and get them back as small cards, that was beautiful) and build interactivity into the text (from search up to interactive tests, video content, image slideshow or even 3D models and micro-applications) it starts creaking under the weight.

Random access speed (when going to a specific page, or to return a list of results) is also nice for a textbook, while it's not very useful for a novel.

nb: the more I think about it, the more those e-textbook sound like modern Hypercard stacks.