The rules seem to have been put in place when these devices came into use. No studies existed on how these devices could interfere with the plane's electronics. It seems the rules remain in place because, well, just because.
I'm not sure exactly how the aircraft is more susceptible to radio interference during takeoff and landing as opposed to when it is at 30,000 feet, flying at nearly 600 mph. While in the air, not only can you use your device, the airline would like you to enable the Wi-Fi radio and pay to use the plane's network. Of course, the flight crew is allowed to, and does, use devices like the iPad during takeoff and landing.
What is the rationale here? Do they think that if there's is a problem in flight there is plenty of time for the pilot to rush back from the cockpit, have everyone turn their devices off, then rush back and pull the plane out of its iPhone-induced uncontrolled nose-dive?
According to Nick Biltin's blog in the New York Times, tests showed a Kindle emits an extremely low amount of radiation. One electronic device allowed by the FAA is the electronic voice recorder. It, too, was tested and found to emit more radiation than a Kindle. When in flight mode, I find it hard to believe that tablets, smartphones, MP3 players, and other e-readers would emit any more than a Kindle does.
Even if these rules were shown--beyond a reasonable doubt--to be of no benefit whatsoever, it would likely take years for the FAA to actually lift the ban. Until it does, though, turn your devices off and stick them in the seatback pocket in front of you. Arguing with the crew about the absurdity of the rule won't do you any good, and the guy next to you has a connecting flight to make.