I don't have to trust Gox. They are in no position to take my funds (my BTC denominated funds that is, they can have a field day with my fiat). Nor is any hacker that manages to compromise their site in a position to swipe my funds. At the same time I have the ability to trade instantly without waiting for six confirmations, as I would have to do if I chose to keep my funds local and only send to Gox each time I want to trade. Most merchants will accept payments from such an address without waiting for confirmations, since Gox will not sign a double spending transaction. Lastly, my money is safe even if a key-logger is installed on my machine.
As shown above with multisig you have to trust:
- Gox's javascript - Which you must verify each and every time you connect as it could be different from last time.
- Gox not to dissapear - Sure they should have emergency / backup procedures, but you can't prove they are doing this properly.
- Your local PC not to be compromised. (As you rightly point out, yubikeys don't really help either way)
This is better than without multisig since without multisig you have to trust all the above, plus:
- Gox itself doesn't get compromised.
But here's the thing. Blockchain.info keeps server side records encrypted with a client side password. However additionally you can backup your encrypted data locally, so if blockchain.info disappears you can still get your money. In this situation you only need to trust:
- Blockchain.info's javascript - each and every time.
- Your local PC not to be compromised. (Again yubikeys don't really help)
Still not perfect, but better than multsig, no?