I believe that a lot of people use early disembarkation not to treat the cruise as a ferry but because a port stop is way more convenient to get home instead of the final port.
For example, we are going to book a Bahamas cruise with MIL, DS and BIL. The one that seems promising is a seven day Serenade cruise from Tampa in February. The sister and her husband live in Nassau and the MIL will stay with them and visit for a few weeks after the cruise.
It would be really nice if the three of them could just disembark on the second to last day in Nassau and go home rather than staying on for essentially one more day and then flying home from Tampa with the attendant costs that will bring. Obviously , they still have to get to Tampa at the start but It would save quite a bit of time and money at the end. It would be well worth it for them to pay a reasonable fee to Royal to be able to do this.