If you cant trust the other party, the only recourse is the full legal contract. If you can trust the party then the handshake and the intention is enough.
Further more even handshake deals made by trustworthy agents with the best intentions sometimes fall apart before the full legal contract is signed. This will never go away.
Bottomline, handshake deals work in situations where the reputation of the individuals with each other is more valuable than the benefit of breaking a given handshake deal. Its just game theory really.
Further if the protocol did get buy in and become widespread, it will eventually reach a point where there are true legal ramifications for breaking a handshake deal. At which point everyone will (and should) refuse to make these deals for the same reason no one signs million dollar contracts without having the lawyers review the details.