I would say "The brain prefers e-ink vs backlight" instead. I love reading on e-ink for long periods compared to a LCD backlit screen. With e-ink I don't think there is much difference compared to paper.
Another factor if one has their tablet loaded with apps, they may fall into the "check email" every 3 seconds rather than read their book. Another reason for e-ink as most readers are just that readers, without a lot of other features. Although one think I like about the the B&N Nook simple touch is its hackablity (and its expand ablity), but I still prefer the format of my nook classic ultimately. Depends on the person, some people prefer to be on a less featured device, some prefer to have the features but can ignore them when reading a book.