On every Carnival cruise we've taken since 2003 they have abandoned the use of fountain soda. All soda comes from cans, even that for the soda cards. Of course, it is not a full can per serving when using the card - more like half a can or less. I simply cannot endorse adults buying the soda card. For kids it is the convenience, even if you don't end up getting your money's worth in product.
My son has also used his card to get more than soda. He likes Carnival's fruit punch, which is made by the bartenders (orange juice, pineapple juice, grenadine, and whiskey sour mix). He has always been able to get this without incurring an extra charge. It may have to do with my familiarity with the bartenders!