Having to make a PA after being cleared for takeoff is one extra diversion from what the pilots should be thinking about/monitoring. ( Correct runway, flaps are set, correct intersection for performance data, what thrust should I see , where is the wind coming from etc etc, what was that heading he just gave me?) . Same as flicking lights on etc.Rubbish SOP's that don't account for the fact that the human brain can only do one thing at a time. best way is to have the cabin manager ding the flight deck when the cabin is secure and that's the end of it.