Aircraft go through all sorts of RF interference testing before they are airworthy. The reason phones must be turned off is because it's virtually impossible for the FAA (or other governing body) to test every single device to make sure it doesn't cause issues.

Since some carriers now allow mobile phones and wifi in flight, I think it's pretty obvious they aren't a risk.

The main reason to have PED's turned off on takeoff/landing is to keep pax focused and ready in case of emergency.

I'd be willing to bet that in the OP's scenario, whatever the pax was using wasn't a mobile phone. They could have easily brought a VHF transmitter on board.

The majority of those issues are pax behavior issues. Actually, very few, perhaps none have anything to do with phones causing issues.

That being said, I found this one most interesting:

Synopsis
A passenger's Kindle Reader was crushed when a First Class Sleeper seat was
retracted causing a fire and smoke. A Flight Attendant put the fire out while the
flight crew accomplished the Smoke and Fire Checklist, including declaring an
emergency.