> * Mental context switch between server/client languages.

Not sure I follow. The client-side Javascript you write will probably interact with the DOM API. The server-side Javascript will probably interact with the filesystem of the server, it is BYOD (Bring Your Own DOM). Plus, no matter what, you will always have that switch between client/server because that switch is what defines client/server.

> * Better code reuse, i.e. the same model class can be used on the server and client side.

Does anyone actually do this? True, it is the same language, but it is not the same environment.