As one from the pointy end I will always let the pax use there phones if we are delayed providing the aircraft is not about to move.
There is a whole load of rubbish spouted about mobile phones being a risk in terms of induced EMF being a fire hazard, I find it hard to swallow that the 0.25W UHF TX from a mobile being a hazard when most airliners have 5 DME units putting out 200W UHF TX whenever the radio bus is powerd.
In my opinion most of the regulations have been writen by cabin crew management who take the stance that if they don't understand it they ban it !.