SWA will have an limited number of planes that 'go out and come right back." It will mostly involve end of the line stations that do not have through flights. Example of a line might be DAL/HOU/CRP/HOU/DAL. There might be a few lines per day that shuttle a couple of times between HOU and DAL but most will then go somewhere else. So, you might have a SJC/BUR/SJC.
As others have said, WN is not going to want to cancel just because a flight is lightly booked BUT if the MUST cancel one for MX reasons, or because a flight is very late and this will get the line back on time, they'll likely chose a lightly booked round trip.