Sometimes, if you're really screwed, you may not be able to mount the partition with vi. ed is almost always on Unix, though. (Knowing how to open a file, find and change a few lines, and resave is probably good enough for emergency situations.)
Also, it's worth learning ed because the ex prompt in vim shares most of its functionality, and because sooner or later you'll realize how lucky you are that you don't have to work on a teletype. :)
Your comment made me smile. I had a thought of a fictional programmer so editor-centric, they decide on their programming language based on which one will help them improve their skills at vi. (Which is a misinterpretation of your comment. vi has roots in ed, so some of the cool esoteric stuff is invoked in the same way.)
The main reason to learn vi is that it is installed by default on so many systems, including most ancient Unixes.
When your task is to edit a config file and you don't have time/permissions to install your editor of preference.... vi is the only choice.
Emacs and others may not be present on a system when you really need it.
A basic competence in vi is essential if you're going to deal with Unix. Even if you prefer Emacs (like me) or other myriad tools.