That would stop phishing attacks dead in their tracks, which the community has been having problems with lately. It would be like multi-sig bitcoin transactions. Maybe you need to PGP sign every bitcoin spend too. Well, security is hard, and that would make the barrier to entry into these markets much higher. It would work great until the first person loses their PGP key, and then we're back to relying on the good will of an anonymous admin somewhere in cyberspace. I could make a locked down, multiple layers of security, hacker-proof web app, but I couldn't make one that was easy for regular people to use. That's the hard problem.