An E-Reader display might be even more attractive as you could just exchange it easily if you know you go on a longer trip by train or such. That way you wouldn’t need to have an external e-reader device. – Which again might be a problem as e-reader manufacturers probably aren’t interested in a deal that decrease their own profits:
Anyway, I like the idea a lot.
[EDIT: I just thought about it a little more. It’s not that easy to simply exchange the screen with a reflective (e-ink) one. There needs to be software that recalculates the OS’s GUI in order to be able to use it. Hardware needs are probably also different. Sounds quite complicated to me 
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[(sorry for another) EDIT: Here’s an already existing concept for a smartphone with an e-ink screen as a second display: The YotaPhone: An Android smartphone with an e-ink second display - Video - CNET]