It's not quite impossible - to defeat 2 of 3 signatures, you just have to be 2 of the parties to the transaction. In this case, if I've built a massive reputation as a fair escrow agent, I can then pose as a seller of something(s) expensive. Once the money is escrowed, I release the funds and disappear. Having 2 of 3 signatures for escrow is certainly a better situation, though, presuming it doesn't lead to a false sense of security.