Thats true. I don't think anyone yet has thought about that. Nonetheless, it's a interesting idea. For me, I don't know if I'm comfortable with taking a picture of my card. But who knows.
card.io shouldn't be used for in-app purchases (Apple's StoreKit works quite well there), but for mobile commerce: selling goods, travel purchases, daily deal sites, local services, and the like. These are all places where commerce is taking place outside of the App Store.