As some have mentioned, the front light not only facilitates reading in a variety of settings and situations, but it also removes the 'grayish/greenish' tint that e-ink is infamous for.
Personally, this never really bothered me - I've been an e-ink adopter since the Sony PRS 500 and have seen the screens get progressively faster and less dark.
Now, with a front-lit technology in place, these earlier screens that I found more than readable are now almost unreadable.
I could never go back, although I still have a Pocketbook 360 that sits in a closet unloved and alone.
Don