Absolutely the safety demo should be performed on every flight. Even as a relatively knowledgeable flier, I always get the card out and have a quick squint, have a listen to the FA's - Hartington's comment about subtle differences is right.. Also, chances are that quite a few flights have pax on board who have never flown before, and therefore won't have a clue how to operate the oxy masks, doors, escape chutes etc.
It's not just aircraft, all visitiors to our site are shown where the emergency exits are and what to do in the case of an emergency. The fact they've been in thousands of similar buildings in the past doesn't mean they know what to do in ours.
It's just common sense to tell people these things!