Who cares about what they're backed with? If it's not bitcoin, then you don't get the benefits of bitcoin.
The only difference between a normal debit card and a bitcoin-backed debit card is that the bitcoin backing layer is more expensive than the usual one.
You get at least one benefit. You can keep your large-scale financial life as private as you like. Not as well as with the newer privacy-focused cryptocurrencies, of course.
And that was arguably a key goal for Bitcoin. It came up in the shadow of ecash and e-gold. Although it's not as decentralized as many expected, it's also far less vulnerable to government takedown than ecash and e-gold were. But sadly, it's no less vulnerable to speculation than any other small currency has been.
Or maybe you'd buy a few month's worth of burgers.