Yes, it's necessary. Some of the points raised you'll already be familiar with but the real point of a safety briefing is to make people aware of the subject of safety and to think about it for a moment or two. Those listening will look around and locate their nearest emergency exit, it doesn't matter how frequent a traveller you may be, if you don't know exactly where your nearest exit is located then you are reducing your chances of survival in certain emergency situations; think about the recent high speed evacuations of airplanes which have experienced emergency situations on landing (fire)
I also agree with Hartington's comments on pure courtesy. How annoyed do you get when presenting something at a meeting and some people don't pay attention, annoying isn't it ? Also, hardly comparable in value to a safety briefing.