Rarely. This subject has been discussed regularly. Seems someone found some value in booking some cruises through II that offered a discount and didn't require depositing a week. I'm not an II member so couldn't test it to see if it was a better discount than say, Vacationstogo's cash price.
So far, anytime I have put my MF cost up against the discount offered by a cruise discounter, the only time I've come out ahead (using RCI) would be to book 4 cabins.
Jim