I can't say for sure if you can get soda without the tumbler (but I thought you could) because we passed on the soda stamp on our RCI cruise. However, on Carnival, I can say you don't need the tumbler. In fact I think they charge a little more for the tumbler with all you can drink pass than without. What a deal, pay more and then have to lug around a cup for your drinks...
I'm with the others on this. Its not worth the cost. Especially if you are going to be drinking anything else (hard drinks or soft). If you have juice/coffee for breakfast and wine or drink at dinner, there isn't enough time in the day to drink 4 more glasses of soda to make it pay off. IF you are a non-drinker, you can even get by with the iced tea/lemon aide without a card and save the money. Even if you have a couple of sodas at $1.50 each a day, you are ahead of the game.
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