When I arrived at the Jacksonville airport, one of my fellow golfers related his experience flying in that morning. Seems that his flight had fewer than 20 passengers booked on it. He told his buddy, "You watch. They'll cancel this flight."
Sure enough, about 15 minutes after his prediction it was announced that the flight was canceled due to a problem with the hydraulic system. He rolled his eyes as he told this part of the story. As far as he was concerned, the "hydraulic problem" was an obvious ruse by the airline to avoid a money-losing flight.
I tried to set him straight on this, but I could tell that there was no way I could convince him that we don't cancel flights because of light loads. There's a good reason for this: It's because that plane is supposed to get somewhere to make another flight, and then another one after that.
I've flown for two airlines, and not once did I have a flight canceled because of a light load. In fact, I've had several flights where the crew outnumbered the passengers. I remember having three passengers from LGA to DCA on an MD-88. We had two pilots and three flight attendants on board (one per passenger...pretty good ratio for them!)