It remained a valuable skill into the 1990s when many UNIX machines still had a separate /usr partition.

Many times I'd have to fix a server with a configuration issue causing /usr to not mount correctly on boot. When you're at a single-user shell and no /usr/bin/vi is available suddenly knowing the basics of /bin/ed becomes very useful.

Of course these days most UNIXes moved away from a separate /usr partition so if the root filesystem mounts you have a full-featured vim.