I've been one of very few pax on a plane (remember one LAS-PHX flight on HP about 5 years ago with maybe 5 pax), and I've also been scheduled for one of those light loaded flights that was cancelled while the aircraft was reassigned elsewhere. Generally, they do not cancel simply for a light load; a Dec MUC-PHL flight I was on had maybe 75 pax total. But if something happens with a more full flight going elsewhere (maintenance for example), all bets are off.