It could be. The problem, as I see it, is that to be used as a currency, we can have the wild swings in price. BTC today is a prime example. If you bought something at 8, it was priced much differently than at noon. The price shot up so much percentage wise that the pricing would have had a major impact on either the buyer or seller (or both).
Steemit seems to have done it a bit better by pegging the currency to the USD so at least the swings in the power isnt that great. Of course the steem itself could go through wild swings.
I think that in case of a debit card it's not that big of an issue, you buy a product with a crypto, the seller receives fiat-currency. The holder of the card obviously wants to spend crypto even though the price might go up right after he spent them.
In my opinion it's better to change all fiat you are going to spend anyway into crypto, put them in debit-card wallet, after a month or so you will already buy more with it then if you would have kept it in fiat.