If you can find some specific scientific argument why e-ink is "an essential requirement" (besides personal choice) then it's just cargo cult.
I listed fairly accurately were e-ink is better, namely reading in sunlight. And battery life, which I forgot.
For reading inside, it's advantages of yore, like resolution have been matched by hi-dpi screens. The only other advantage is better default contrast, which is a configurable setting on a tablet.