Comment by AStonesThrow

Perhaps EDs are a concern, but mostly what terrorists actually use phones for is remote detonation. A feature phone is the ideal sort of radio transmitter, modified or not, to send a remote command to detonate explosives. If security is really scrutinizing carry-on luggage, then perhaps the strategy moves to placing a bomb in checked baggage, and carrying only the trigger through the checkpoint.

So if you force someone to turn on their phone or laptop at security, you will hopefully force the decision point; even if the inspector cannot tell the difference, a terrorist is going to get real nervous and jumpy around activating the thing they intend to use as a detonator, in contrast to some businessdude activating his very ordinary mobile device.