Probably you are right when you are refering to Word docs.
However, we have 6" screen which is not too long even in a landscape mode
From a few pics I have seen, the margins on Cybook were way too big for my liking, on the other hand I always leave margins, albeit much smaller. I don't like when the text is reaching reader's frame, I prefer to have a bit of white space on left and right. And at the bottom I have footers.
If I ever get 9.7" screen, I am going to leave bigger margins, I promise