I speak as a teacher here
A book reader is a tool, certainly, but it's not the only tool---Kid Pix has been around for decades, but we still give them crayons; calculators have been around for ages but we still teach them how to add sums; Google Earth is very handy but we still teach them how to read a globe. You have to walk before you can run, with any skill, and just like we don't introduce the calculators right away because we want them to have the basic concepts of numeracy, we don't introduce the readers or other tools until they understand the basic concepts of literacy, and not all of those concepts involve the mechanical reading of words. There are concepts such as the way the pages go, that in English we read from left to right (this is not true for every language), that pictures or illustrations can offer textual clues to help us read etc. A lot goes on before they get to the point of reading independently.
Middle school, I can see a reader replacing a bagful of heavy textbooks. But primary school? I don't think so. They aren't gentle enough with technology, and they aren't ready yet either. As I said, teaching reading to little kids is not just about the letters and the words, it's about the whole concept of reading, and they do need a tactile experience for that, just as we still give them little plastic teddy bears to manipulate when they learn to count (in fact, there is a whole genre of educational product called the 'manipulative' that caters to this very purpose). A book is a manipulative. An ebook reader is a calculator. And the advance of technology won't change that.