When I watch TV in the kitchen, everytime the washing-machine or the fridge engine starts, I see an interference in the TV. It's just a fraction of a second, but it's noticeable. Neither my washing-machine nor my fridge have wifi or 3g connection
I don't think it's too far-fetched assuming that every electronic device could cause some interference in a plane's instruments, and it's a risk not worth facing. Of course, an ebook reader (at least one without wireless connection) is probably one of the less "dangerous" devices, but when setting safety rules for arbitrary groups of people, it's often better to just set clear-cut strict rules and not deal with possible case-by-case exceptions, if everyone starts saying "but look, my device's user guide says..." it would be chaos.
On the other hand, I'm not sure there has been enough serious testing of what devices actually do or don't cause interferences with the plane's instruments, and these should be designed to work properly in an environment full of electronic devices (even if you ask all passengers to turn their phones off, I'm sure at least one phone in every flight remains turned on).