I'm with the OP - I don't know the ins and outs of the real technical risks, but given the clear and well-known instruction and practice is that phones go to off/flight safe once doors close, I am not prepared to take even an infinitesimal degree of risk to myself, my family or anyone else on the plane just so some berk can ignore the rules and use their phone - I have made the same request of fellow passengers when necessary...(most recently, a guy on a voice call as we lined up for the take-off runway at LHR...).