Also, each aircraft is subtly different.
There have been too many occasions when people have got out of wrecks *because* they listened to the demo. I'm not a nervous flyer and I do sufficient flying that I could qualify as one of those who "don't need" to listen. But, I always listen and I always have a quick think about how I would evacuate if the need arose.
There is also a question of courtesy. The crew have no choice - they have to complete the demo. Depending on the airline they may do it by the book or they may add some personality but the fact is they stand there and do it for my benefit (yes I know they have to, but in the end it is for my benefit) and I feel it is only common courtesy to show some appreciation for their efforts by watching.