I wouldn't travel to a cruise that short, but if I lived in or near a port city, I'd do them evey chance I get. Especially those Princess repo cruises up and down the west coast. Kind of like the cruise version of a mileage run. 5 cruises makes platinum status in Captain's Circle, and it doesn't matter the length of the cruise LOL.

Or, if there was a cruise to nowhere happenning right before or after a longer scheduled cruise from that port, I'd book them back to back, and make a longer cruise out of it. (Something like the Oct 11 Carnival Legend out of New York. 8 day Caribbean, followed by the Oct 19 2 day to nowhere)