In the long run, it's cheaper to just pay cash for the cruise than to use points. We used points in 2001 to go on the cruise (4 nights) and we used all our current year points and borrowed points as well. We wound up buying more points while we were at WDW on that trip. We bought more again the next year.
We are going cruising (7 nights) this coming spring. We're paying case because, again, we would use all our current year points and borrow from the next. For three of us in a Cat 6 room it was a lot of points.