I had to learn ed years ago to configure a device that only had a serial port (it didn't run unix or termcap, but they ported ed for configuration purposes). Every once in a while my machine gets bogged down enough that I can pull up ed and make changes faster than vi (vim?) can load.
I had to use ed once in a very minimal rescue environment. I had never used it before, and I also only know vi in a very basic way (emacs is my daily driver).
It took some study of man pages and trial and error to achieve what I needed, but I was able to get it done.
