In my opinion, using e-ink screen as the keyboard is mostly the same as using on-screen keyboard due to the same flat surface without any perceptible forms which make typing on hardware keyboard unique. Noticeable advantages are not covering the main screen, (maybe) power saving and as mentioned in the previous post making a single keyboard across a wider area. In order to take advantages of both the e-ink and the hardware keyboard every single key would have to have a small e-ink screen. It might be very convenient for languages using writing system other than Latin but with signs meaning more or less a single phone (phonetics) such as Russian, Ukrainian, Greek or Georgian as it might be easy to switch between (noticeable different) key layouts.