Passenger Psychology.
Radeng - An argument maybe, but obviously not important enough to warrant a policy in which it is mandatory everyone HAS to observe the Safety Demo. A passenger knowledgeable or not may still freeze up in shock etc...in a real evacuation and respond differently anyway.
As for passengers at overwings, exits etc... on my airline we brief those passengers individually and make sure you they understand the instructions and to read the information in front of them and safety card. Again we don't force anyone and we are not happy we move them. We don't even have to move non English speaking passengers from sitting next to the exit.