Consider a 7-night cruise on the Disney Magic. A 3-night cruise is really just two full days. That's a lot of points for such a short cruise. And, frankly, the least enjoyable part of a cruise is embarkation and debarkation. The 7-night cruise goes to better ports, provides far more things to do, and really lets you enjoy the cruise experience
A land/sea combo might be good for someone who is doing this as "once in a lifetime" experience, and wants to see WDW and take a cruise on a single, rushed trip. But as a
DVC member you'll be back at WDW another time.
As to whether the cruise is a good use of DVC points, you'll have to make that determination for yourself.
It was a great use of DVC points for us. For the Disney Magic 3/23/02 cruise, we paid 578 points for two adults and two children in a category 6 stateroom. If I had booked with cash back in May 2001, the same stateroom would have cost $6,055 (which includes the early booking savings) according to
Disney Cruise Line's online reservation system. That's because the week of 3/23/02 was a very popular week. If we had gone during most other regular season weeks, the cash price would have been lower, but the number of points would have been the same. I'm sure the cash price for the cruise would have been somewhat less from a discounter.
This works out to a value of around $10 per point.
Instead of spending around $6,000, I spent a $75 fee plus the money we were spending anyway on DVC. Of course, in either case you have to plan for tips, alcohol, some non-alcoholic beverages, shore expenses, and shopping. But, except for the tips -- which is how your servers and stateroom host are paid (and which thus really aren't optional) -- the other expenses can be as little or as much as you want.
Does this mean we couldn't enjoy the Central Florida attractions for a couple of years because we used all our points for the cruise? The answer is no. Because we own other timeshares, we picked up some bonus weeks in very nice 2BR Marriott timeshares for $199/week. True, they're not quite as wonderful as DVC resorts, but they come close.