Being able to pull out any book, anywhere, any time, is not useful? What happens at night in your electricityless world, are you reading by candlelight? Or you can just light the page you just read on fire, that way your book is also your fuel.

As someone who reads quickly a lot on planes (and sees a lot of others doing the same) the ability to have multiple books on a Kindle is much more important than lack of access to electricity for a few days.

(I also have a large physical book library.)

e-readers can go for days to weeks on a full charge depending on how often you use them. Definitely much longer than the average smartphone.