Well one use case as was saying Aaron would be to renmame a bunch of variables.
Another, for me would be to jump around the file with keyboard shortcuts, use multiple cursors, grep it and maybe keep my own git logs.
Maybe this is just an issue for people who are still learning like me, but I would really find it easier to develop a notebook with my editor rather than in the navigator.
Now, I don’t think there are many people using emacs as their preferred editor so your probably right any way.
Edit: and I didn’t know about Codemirror before reading this post.