As you said, the problem with involving a third party with each payment is that they could eventually go out of business (or simply have an outage at a certain point in time). If all they do is publish a list of Bitcoin addresses that have double-spend-proof smartcards behind their private keys, they can't lock you out of your own money (by refusing or being unable to sign the transaction when requested). This removes them from a position of power (they can't block your funds or refuse to allow you to spend them for political or legal reasons).

Also, if they ever go out of business, you can still use your smartcard-backed wallet as a regular wallet (it would still sign transactions and they can still be verified as double-spend-proof by anyone that still has the list of public keys they've published). New Bitcoin keys could be added to the wallet (or rather generated by the wallet) and it would then function as a standard Bitcoin wallet since there would be no one to update the public key list (vendor went out of business).

So you don't entrust a third party with your funds and give them the ability to block your transactions. You still have complete control over where and when you pay, you just don't get access to your keys. I suspect that most non-technical users will not care much - as long as they can quickly and easily pay for their transactions. Just like most of them don't memorize credit card numbers or account numbers, all they care is that the transactions go through quickly and without much involvement from their end.