If they're going to claim that phones or other devices cause interference, they should at least be specific about whether they were in airplane mode or not. I can see the point of turning off cel and wifi, but when they insist on turning off devices like old ipods that have no wireless communication, that's just stupid.
There really isn't time during prep for take-off and landing for the flight attendants to go around to each passenger using an electronic device and have them hand it over for inspection.
Since we know that many passengers do turn their electronics back on as soon as the flight attendant is out of sight, we know that those passengers at least cannot be trusted. The most practical way to enforce the rule (as long as it exists) is to have all passengers turn off all electronic devices and put them away.
There are other ways, of course:
Install detectors to triangulate the position of passengers who are transmitting wifi, bluetooth, or cell phone signals during times when those are prohibited. Then the flight attendants can speak directly to the offender and ask him or her to turn off the device (or at least the signal). Passengers who do not comply or who pretend to comply and then turn them back on again can be placed on a no-fly list.