That's something I've noticed too. I've no problem with prototypical operation at exhibitions but there needs to be something happening all the time if exhibitors expect to hold people's attention for any amount of time.
A small loop with a train constantly running will have more interest for the casual observer than timetable-driven scenic masterpiece that has minutes elapsing between each movement.
The type of operation that David describes in his first post is perfect if you want to keep a layout constantly on the move.
To drift off topic a little; this is where I think sound equipped stock comes into its own.
It allows you to drive the locos at protypical shunting speed without it appear TOO slow to a casual audience. You can also reproduce most of the action that goes with it like coupling, brake hoses, charging brake lines etc.