You can turn off the backlight for an LCD and rely on it being reflective, but the contrast is very poor since at least half the light is blocked.

In this application I see the use of e-ink as being:

- Maybe more readable if you're really fussy.

- Lowers overall power use.

The prior is super weak, LCDs do fine as long as they're not in direct sun.

The second is a good reason, but not a very compelling one in terms of economics yet.

The place these make sense is battery powered devices. If it's plugged in it is a gimmick or someone really dedicated to energy efficiency.