The view from the rainforest room of Nassau is actually very nice. We did a morning of rainforest room and palo brunch, and afternoon at the pool, and pirate night in the evening on Nassau day. Then next day is all
castaway cay. I definitely feel it is plenty of relaxation and fun in three days. And the restaurants are all great--they just do the three signature menus, so there are no bleh nights like I hear pirate night menu is on the longer cruises. As someone with seasickness, I actually much prefer being in port both days to avoid the rocking, so to each his own.
To OP: I think whether the price is worth it largely boils down to airfare. We are a two hour flight away on Southwest, which is generally not too bad price wise. We find the actual cruise prices are more or less the same per-night cost as a longer cruise (unless you are going on a holiday weekend when they can be higher), so the only thing that makes the per night cost more pricey is the fact that you are paying the same airfare for a shorter trip. If I was coming from the west coast or internationally, I agree it might not be worth it.
The other thing to consider is that when comparing the 3-day and 4-day-or-longer cruises is that the 3-day is really the only option that allows you to avoid having to take a full week off work. That is huge for us as my company is fairly restrictive when it comes to vacations, but a long weekend is much easier to swing. It also means less school missed for families with kids. So whether it is "worth it" is probably also a factor of the value of your vacation days and how many you can take. If I could take a full week, I'd do a 7-day or a back-to-back 3-day/4-day every time, but I can't.