What if.... what if Carnival actually started to take onboard spend into account? I'd bet that some of those "freeloaders" are actually contributing more to the bottom line than you might think.
I, for one, have sailed on several free/reduced rate cruises. In some cases, I've left with several times less $ than if I had paid for the cruise in full and stayed away from games of chance.
FWIW, many of our cruises on the way to Diamond (yes, we were already "there" too when the program started in the summer of '12) were in suites. That's earned us nothing in Carnival land...
Tom