Originally Posted by jaffab
But are Boeing or Airbus really making planes which will crash because there is some very minor output from an ebook or similar device?
Of course not. There are some radio based navigation aids that can be confused by radio receivers for nearby bands. The use of TV receivers on flights is a particular problem. These radio navigation devices are backups for GPS, the preferred navigation aid.
The question you have to ask yourself is "Do you want to presume that your flight attendant and/or fellow passenger is a knowledgeable electronics engineer?" If you can say yes, then you can trust them to use their engineering expertise to tell folks what to turn off. Alas, if you can say yes I'm not comfortable with you being in charge of air travel.
These people have absolutely no idea how the iGizmo in front of them works. The rules need to be simple to explain, simple to understand, and simple to check. That's more important than that they are optimal. The "If it has an off switch, then it needs to be off" rule meets all those criteria.
That said, I am an engineer, and I read my Sony Reader all the time during takeoff and landing. However, I don't let people see me because I respect that the flight attendant's job is hard enough without "But you're letting him read."