The following is an example of whether to book using points. Jan. 2003 we booked using points for two adults (168). It was a four day cruise on the Disney Wonder. The day after we booked the cruise it was sharply discounted. The cost per person was $399 and we would have received a $100 ship credit. This was an excellent cash price. If you rented your points at $10 per point the rental income would have been $1680. As you can see you can come out ahead rather than booking using points. Also, you have to include I think the adminitrative cost of booking is $95--I think we paid $75.
The cruise was great and I would recommend it. However, using points may not be the best option. There maybe other circumstances in peak times when you get the value using your points. In the future if I intend to take a cruise I plan on considering renting the points and paying cash.
Good luck and enjoy!