MERN is just a starter project that helps you get started with isomorphic apps. Adding things like sidebar navs, forum posts, profiles etc (if that's what one wants to do) is up to the developers.
It's a great start, but if you can show people best practices, for a basic application, it will be 10 times more usable, since anybody that wants to build an app, needs nav, user db, registration.
@surfyogi I agree. That's why we have plans to build separate example projects with auth, navs etc. Users can then use our mern-cli to clone the appropriate project. If you want to build one, let us know and we can have a discussion.