It can be worth it if you don't mind spending time comparison shopping. The good news is that they can take 3 years of points if you plan it out. If you choose a 2016 cruise, they will take your 2015, 2016 and 2017 points if you need that many.
I never booked a cruise that way, but there was one time that I almost did. It was Disney and it was the week the schedule came out. At that time, the cost was comparable to what I'd pay if I paid cash for the cruise. The price went up and up and up after that and wasn't worth it to me to book it later on.
You won't find a bargain, but if you don't plan on using the points for HGVC, it's better than not using the points at all. You factor in the cost of your MFs + whatever extra charges there are for booking. Then, look up same cruise on the cruise ship's website to see what you'd pay if you didn't use points. Many times the cruise ship's website will offer extra specials like free drinks or free gratuities. So, factor those in, also.