My kids used theirs to get some non-alcholic ship drinks, but you should ask when ordering if it will cover anything non-soda.
As far as if it is worth it or not, it depends on how much soda you will consume. Since you won't be drinking the hard stuff, I would guess it is worth it. For adults, the card runs $7 a day (and that includes tips, but I found that servers tend to "disappear" and never return if you order with the card, so maybe try a little extra tip?) If you figure a glass of soda runs about $2 plus tip, you can do the math. The card WILL NOT get you bottled sodas, only glasses and it is usable in the dinning rooms. It is good for my kids, but I found that because I would have wine or mixed drinks with dinner and at the shows, that I never really used it enough to make it worth it. But in your situation, it is probably good. Get it on your first day.. they normally will have a table set up on the pool deck somewhere sell them.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007