Considering turning off cellphones negates one from using it for about 20 minutes total during takeoff and landing, it's not much to ask to take out a variable that was never fully tested.

New devices come out daily and they're only getting stronger in respects to EM output. I rather not be on the flight where a cellphone causes a crash, thus creating a new rule of no cellphones to be carried on to a flight.

For me, a glance at their cellphone and then eye contact with the offender or asking them to turn it off has worked.