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. :)