We just cruised Alaska on (mostly) points. The cash quote for the 2 cabins was just under $12K. I used 1066 points and paid $95 and $1295 for one of the cruisers. It was done in one phone call and we got the cruise and cabins we wanted.
I think there are two things to keep in mind:
1. The number of points needed--Renting out 300 points or so is different from having to rent out 1066 points. That would have required multiple rentals (and possibly multiple headaches).
2. The total cost--I am willing to pay a little more for the convenience (say if the points needed were 1000 but the cruise only cost $9K). But I wouldn't do it in every circumstance.
#2 is true in any circumstance, not just cruising. We have never used our points for easter or christmas week in wdw either because we don't feel we get the value we want from the points. Cruising is no different. Many people here used to pay cash for weekend nights for the same reason. I consider this all the same issue. But sometimes the points required for a cruise are right in line with the value of them.
We would not have done the Alaska cruise if we had to pay cash, and it was one of the best vacations we ever had. We felt (this time) the points were a good way to cruise.