My impression was that most pax who didn't watch the brief were switched off because they'd seen it all before, or thought they had. Or - and these were the worst - they wanted to look like they'd seen it all before.
I did often wonder if some announcement on the PA beforehand, to the effect that it is simply a courtesy to look up from your paper when the CC are doing an important part of their job, might help. That approach would at least let the posers save face. And quite apart from the safety aspects you so rightly emphasise Juud, I always felt it was just plain rude (or ruud) patently to ignore someone clamouring for your attention a couple of meters away.