Even if they publish their server code there is no way for anyone to verify that it's the code they are actually running and it would be just a PR move. If the client implementation is good there should be no way that the server can compromise any message.
It's a step toward people running their own servers, either federated with Keybase proper, or just as a personal instance. That would be valuable for quite a number of enthusiasts. Federation (like email/XMPP) is a very reasonable feature for any forward-looking communication platform.
Keybase's target is to become a central identity point. Other features (like team chat and git repos) are made to showcase what you could do with that.
Okay, and that's exactly the kind of reasoned response that's appropriate. What's not appropriate is implying the request is simply unfounded because of its source. It's not charitable.