I'm guessing that the whole RF interference explanation comes from the days when electronics used far more power and operated at frequencies much closer to aviation communications frequencies. That being said, there are still huge liability concerns if modern device should interfere with the communications or instrumentation of a modern airplane so nobody wants to go out on the limb and say that using the stuff during take-off and landing is okay. I would also suggest that anyone who can't put down their electronics from the time that the plane leaves the gate until the seatbelt light turns off are probably thinking about themselves when they say that there are no safety concerns, rather than about the potential safety concerns themselves. (That is especially true as others have also pointed out other concerns: distractibility while giving instructions to passengers and loose objects in the cabin.)