I believe it's mostly for passenger management reasons. Like you said, having that many people talking on the phone so close to each other (and in a place where you can't get away from an annoying person), would be too disruptive. Cellphones on airplanes also can briefly cause some havoc with the towers near the airport, since the phones are moving through different cells much more quickly than the network was designed for. But that's a minor annoyance to cell providers and not a safety problem.
As for turning music players off during takeoff and landing, I think it's more of a passenger safety thing. The vast majority of airplane crashes occur at the start or end of a flight. The flight crew needs to be able to communicate with passengers if something were to happen. And a passenger who's rocking out on headphones may not realize that he has to move quickly or that he's blocking another passenger's path to safety. The same thing could be achieved if you had the headphones on, but no music playing, but the only way that flight attendants can know you aren't listening is to have you take the headphones off.
As yg17 noted, if a cellphone on a plane were really that dangerous, it would have happened by accident many times by now. If there really were a chance that it could bring the plane down, they either wouldn't let you bring a cellphone on the plane at all, or would confiscate them during the flight, turn them off, then gave them back to us after we landed. I remember just after landing once, the guy next to me pulled out his phone to turn it back on, but realized that he had accidentally left it on during the whole flight. The two of us joked that he could have killed us all (which another woman nearby didn't find funny at all). If the airlines told everyone they had to leave their cellphones off just for the courtesy of others, no one would do it. But make it a potential safety issue and people will grudgingly put their phones away.
