How about two detachable screens? If you only want to take the eink screen with you, you could. If you want both screens, you could.
Maybe you could be reading on eink and in the next screen there could be a book club chat app, so that people could discuss and talk while reading the book.
Maybe you're in the mood to read some comic books, so you only take the lcd screen with you.
There's lots of possibilities.
It certainly will be more affordable that buying separate eink reader and tablet pc.