Example:
A customer walks in to a store and pay using a plastic card that's only link to a certain cryptocurrency (Bitcoin, Dogecoin, NXT, BitsharesX, ect...)
The store takes that plastic card and swipe in the card reader and it deducts the amount in (Bitcoin, Dogecoin, NXT, BitsharesX, ect...).
This is the only way people be able to keep track of the total supply of the coins being distributed.
A card as you describe would have the exact opposite result, since you can't deduct any amount of cryptocurrency from an address without having the private keys. No-one in their right mind would give a merchant their keys. The workaround would then be to have the card linked to an account provided by some "trusted" 3rd party. This means off-chain transactions, and giving these account providers the opportunity to issue more cryptocurrency-denominated IOUs than they actually have currency to back.