jrabbit -
I have to agree with what you've said in terms of not offering discounts to repeat cruisers. There is no reason to.
However, I must respectfully disagree that they couldn't (or shouldn't) do more. One of the other posters mentioned PR - from a public relations standpoint sending postcards about different cruise options, "specialty" cruises such as holidays, etc. and just getting it in front of previous cruisers even if only every 4-6 months would be ideal.
From an ROI standpoint it wouldn't be excessively expensive to mail a card or brochure occasionally. And for those who are not diehard cruisers (we're headed for our 3rd in June), like to take a variety of vacations, etc. it does a world of good to keep the cruise option as a top of the mind option.
Deb