It is usually just the number of ports that you go to. In Galveston, there is a 4 night cruise that only goes to Cozmel whereas the 5 night cruise goes to Cozemel and Calica. The seven night cruise goes to Cancun, Cozemel, and Belize. It depends on where you want to go and how much you want to see.
We have friends who will go on a cruise just to get away and have a holiday.
We on the other hand enjoy the various ports of call and will select a cruise based on that. With the short 3 - 5 day cruises you may only visit 1or 2 ports (unless you take the cruise to nowhere). If I lived within a two hour driving distance to a port, I'm certain we'd take two or three short cruises a year. Many times these are deeply discounted ant they make a nice getaway for a couple of days.
I would also think that the short cruises are great for those who have never cruised and are not sure if they'd enjoy it and they can try it out. And as far as the cruise lines are concerned they are filling a need. Not everyone can afford or wants a 7-10 day cruise. For them these are short and sweet and have a high turn over of passengers. I'll bet that during the summer months when school is out that a 4 day cruise would sell more booze than the 7 day during the winter months.
The shorter the cruise, usually the younger and more 'lively' the passengers are. Quite frankly, now that we have done so many cruises we would not consider a 3 or 4 nighter and a 5 nighter only if the price was very good and we had a reason to go on that particuliar cruise because of friends or something like that. The others are just too short but understand that they are great if your time is limited like many younger working couples.
To me, the 3 day cruises, just aren' t long enough.Thats just long enough to get you started.
The 5-day cruises are good, and the 7- day cruises are even better. The 8-21 day cruises are what I dream of !