Jazzgal, I'm not saying everyone will take this approach but I would bet money that a significant number of folks will look to arrive one or two days in advance of their cruise departing date if not to explore St. Maarten, but just to assure they reach SXM in plenty of time to make the cruise.
You may be right eventually. Based on the number of jets arriving last Saturday, however, I think most folks for that sail were arriving the day the ship sailed.