I totally agree with you Eddy. A lot of people don't want to listen to safety demos because they don't want to think about it, it can be a bit of a sour point for some passengers. I think any discreet "light heartedness" in the safety PA is a good thing as it makes the pax pay attention. Obviously do not take this humour too far, otherwise passengers might not think you take their safety seriously.
I fly so many times and pax ask for reasons for procedures ie. "why do we put our window blinds up for t/o?" or "why can't we listen to our iPods for landing?" A simple explanation (without straying from the point) in my opinion is harmless. I can't think of any other reason a lifejacket has a whistle on it!!!
How many times have you sat through a safety PA and the purser drones....and drones....and drones - everybody switches off and nothing is learned. It's just like teaching kids, lessons have to be interesting or they lose concentration. I'm just as guity at that!