The reason they don’t want you using personal electronics during takeoff and landing is that most personal electronics, even though they’re shielded and not supposed to emit much, emit certain amounts of RFI (Radio Frequency Interference). While one or two devices might not interfere, a lot of them could interfere with aircraft pilot-ground communications, which is critical during takeoff and landing, when the aircraft is most vulnerable.
I was always curious about this. Basically, it takes a group of people (or Alec Baldwins) to interfere and bring down a plane. But it’s ok to use the your phone at the individual level. I feel so much safer now.