After watching Vlad Zamfir's latest video and looking at Casper, I now realize lawyers have a lot to offer in helping with mechanism design. Lawyers who understand incentives and punishments (crime and punishment) know how to structure contracts in such a way to so as to provide the right incentives for human behavior. The mechanism design primarily is what guides the incentives of participants (humans and or bots) to support the continued survival and operation of the network.
A lot of ideas can be borrowed from the solutions found in law. These legal solutions can act as templates for the protocol solutions which can govern distributed networks. Common law for example offers some solutions to common problems such as partnership agreements. Problems such as reputation and credit can also find solutions in law where there are ways.
Legal reasoning is needed
To be precise and specific what I think is needed is an understanding of legal reasoning. It's not so much that we need lawyers in the sense that we need to put real world law onto the blockchain, but we do need legal reasoning to be used in smart contracts and in code, where enforcement can be through fines/penalties and rewards to reputation or and financially. This is what I see Casper doing where enforcement is basic reward and punishment via the token distribution.
External law enforcement is the last resort
When thinking about security, ideally we would want not to rely on any particular government legal system. Ideally we would want to be able to rely completely on the protocol to resolve all disputes and maintain the network. Practically speaking this might not always be possible and so for the last resort it should be that the protocol is protected by all means, both in protocol and out of protocol. Out of protocol could mean using the legal means to protect the functioning of the network as intended by the code and more specifically the formal specification which determines how the code is supposed to work. In that case if programmers mess up and the code doesn't follow the requirements of the specification then the legal process could be used to provide incentive to keep things aligned. Of course that is just one example and probably not a good one because I'm not a lawyer.