I was a MUD coder in a past life (and sort of am still); learning ed is essential for that environment. I'm glad I learned to use it, and gladder still I don't have to use it every day.
It's great for quick fixes and one-offs, but having to engineer with it is rather unpalatable.
I'm curious, why is it essential? I'm a bit of a MUD enthusiast myself, and I've never had to use ed. On the other hand, my favorite MUD's long-form editor (like for newsposts and journal pages) has an ed-like design.
For making changes on the mud itself, there was no other way to write code unless you were an admin and had direct machine access.
There are some vi-like mud editors out there, but they were always wonky when I tried them, and caused more problems than they were worth (weird screen redraws, keys stopped working randomly, etc.). That said, I believe Dead Souls mudlib[1] may ship with one now, but I haven't looked at the lib in a few years, so I can't say for sure.
It's great for quick fixes and one-offs, but having to engineer with it is rather unpalatable.