Lord Lucan
You claim to be an ATPL holder...tell me one thing - are you going to stop running through your checklists before your next take off because you've done them 1000 times before?? I suspect not...the cabin safety briefing is surely there for the same reason as the flight deck checklists...Also, as many others have said before me, its just basic common courtesy to allow the CC to go about their job without interruption and to let those who are sensible enough to watch/listen to the briefing do so.