[vorpalhex]

Airplane mode has two reasons. One reason is because at one point your cell radio risked doing unpleasant things to cellular ground stations, and another reason being that the FAA wanted you to pay attention to your safety demonstration.

Bluetooth was allowed on planes quite some years ago.

[Carpetsmoker]

Another reason to turn on airplane mode – or at least disable mobile – is that your phone will keep searching for a network tower when it's out of reach, which will drain battery quite a bit faster.