You actually can't book with points on the cruise. Instead they give you three days after disembarking to call MS to book the cruise. If the cruise that you are taking is more than 11 months out you still have to call within the 3 days to have them note on your account that you are going to book the cruise at the 11 month window. Then you have to book right at 11 months. They will even tell you which day you need to call back on. I think that they actually give you a 3 day window at that time too.
We booked a second cruise with this incentive. We are in a cat. 6 on the Eastern cruise in May. The points normally are 186, but with the rebooking credit we only paid 174 per person. For two people we saved 24 points. Not bad, it does give you an extra night or two at WDW.
I know that a lot of people will tell you to rent your points and pay in cash, but we rather not go through the hassle. Plus we have talked to a friend of ours who is a tax lawyer and he says that we have to claim the money that we get from this type of rental as income on our taxes. With a 30% tax rate that only comes to $7 a point. That would be only $1218 per person. I wish that I had hung onto the price list for early booking the 7-day cruise. But I know that it was more than $1218 per person in a cat.6.
Then again, we booked our daughters in a cat. 11 for the same cruise and paid cash. We aren't renting points to pay for it though. Really when we compared the price for their cabin it was about the same either way you went. The points are 123 per person which would give us $861 if we rented the points. The early booking price was $799. So we just paid cash and saved the points.
I guess what you have to do is calculate it for yourself and decide if you want to go to the trouble of renting out your points for cash.