I am amazed every time this argument comes up! I was cabin crew for two years, my wife has twenty years in with another airline, and we were both told in training that when you're on a passenger seat, be it for pleasure, for business or deadheading, in or out of uniform, put your book down and afford your colleagues the courtesy of listening to the demo. You're not going to learn anything from it (we can recite those things verbatim!) but that act may just prompt the person sitting next to you to do the same. (Trust me, we actually do fly more than you.)
If it comes to an evacuation (and we all hope it doesn't) Trossie sums it up perfectly above: Someone with a lot more knowledge of the situation than you has made that decision. Get out! Leave everything behind and get out. If you're messing about with your bag you're slowing things down. With cooperation and all exits usable we can get you all out in 90 seconds or less.
When you reach the bottom of the slide, if you have two hands free you can help others not blessed with the same degree of mobility as you. (And I don't mean with their carry ons!) We all know that in an evac the probability of injuries is high. People fall - it happens. Have two hands free and without endangering yourself be ready to help someone who may have fallen to prevent a build up behind.
I will concede one point though. While the airlines continue to charge for checked baggage they are driving more and more into the cabin as carry on.