pnorth,
It sounds like you were getting the cheaper drinks like your well liquors and beers. They are always cheaper, and you do lose money using the coupons on those, and Carnival banks on this when they sell them. However, I'm surprised you couldn't use them to get a souvenir cup. Normally, you can get any drink with the coupon (up to $4.95 before tip), but if you go over, you only pay the difference. So at the very least, maybe you pay an extra $2 out of pocket, but at least you have something to show for it! Depending on your waitstaff, even if you go a little over (say a drink costing $5.50), sometimes they'll let the difference slide.