They do hardly anything at the w/e - the Trislander doesn't fly at all on a Saturday and you will see most a/c parked up for a large proportion of the w/e. You only have to look at the Departures board to see that they ARE grossly underutilised for 2 days out of 7.
Maybe it saves money? Although a/c don't make money sitting on the ground, i suppose if the flight was fairly empty, it would lose more.