Many of us have given you lots of reasons apart from keeping the phone secure in a holder.
So, to recapitulate:-
1. Interference with Radio and Navigation equipment. I would have thought that a radio on would be far more important than having a mobile on. I feel prefectly safe with my mobile phone off in my pocket but I would feel a tad vulnerable if I didn't have a radio.
2. For the same reasons as mobile phone use is banned in the UK whilst driving. It's down to division of attention.
3. It's actually quite a flakey check since most cell site IDs cover quite a large area and therefore, the safety aspects should outweigh the minimal benefit gained of knowing that your somewhere over West Berkshire for example!
Cheers
Whirls