As far as I understand it - the only real downside is the fact that the screen still uses up energy during transmissive mode (i.e. looking like e-ink), albeit very little.
I wonder what the battery duration would look like. If such a device lasts a day in "e-ink mode" (I know the term is not exactly correct), I would be perfectly content with it. Charging it once a day is not a bigge (over night?).