About the only time it's "hassle" free to cancel ONE flight for light loads would be an "out and back" from a hub to an outstation and back. If you cancel the flight with an aircraft NOT returning to it's first departure point, you create way more headaches than it is worth. Not only is the aircraft out of position, but so are the flight attendants and pilots. Just ONE late/cancelled flight can create multiple problems.

Edited to add: Oh yeah, if another more profitable/flight with higher loads has a maintenance issue or whatever, you can expect your lightly loaded flight will be given the shaft in favor of the other flight