As a minimum it is common courtesy to listen to a briefing which is for the benefit of everyone on board.
The aircraft commander also has a legal obligation (for a UK registered aircraft) to ensure that passengers are adequately briefed (even if this is done on his/her behalf by a qualified crew member). The only way to ensure that his/her responsibilities are discharged is to make sure that everyone is paying attention. That is a 'reasonable step' in my book.
53. —(1) Subject to paragraph (2), the commander of an aircraft registered in the United Kingdom shall take all reasonable steps to ensure—
(a) before the aircraft takes off on any flight, that all passengers are made familiar with the position and method of use of emergency exits, safety belts (with diagonal shoulder strap where required to be carried), safety harnesses and (where required to be carried) oxygen equipment, lifejackets and the floor path lighting system and all other devices required by or under this Order and intended for use by passengers individually in the case of an emergency occurring to the aircraft; and
(b) that in an emergency during a flight, all passengers are instructed in the emergency action which they should take.
As others have said, if anyone who thought it unworthy to pay attention during the safety brief gets in my way during any emergency evacuation, I'll be walking over the top of them with no hesitation
