Theoretically it's acting as a layer of security on top of your real debit card. If someone clones your debit card your bank account is directly accessible. If someone clones your Google Wallet debit card, it's a bad situation but not terrible. Of course Google probably won't have the customer service manpower available to deal with such a situation (or you'll end up in email hell) so you're likely still fucked. It's better to just get a credit card.