I checked with Carnival again today and I was told I could not cash these in at the purser's desk. I'll try but the gentlemen was again adamant (at least he thought the policy was a bit odd!). I was again told that I could use them on another cruise.
If you are going on a cruise soon and might be getting these, you may want to get them early so they'll actually be of use on the ship.
Maybe enough people will complain and Carnival will change this CRAZY policy.
Thanks for all your replies!!