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For me its purely the reading in bed in the dark thing. I got a backlit kindle and actually I've found it totally fine reading in darkness, just with the backlight. No complaints about not being able to sleep from Elle - everyones happy.
I've quite often finished one book of a series, bought the next and then carried on straight away, which is pretty great to be able to do all while lying in bed.
I don't get why carrying a book is harder than carrying a kindle? I'd be more worried about breaking the kindle.