I used to use ed when all I had was a single terminal screen/window and a compiler error message. Ed doesn't clear the screen, so you can still see the message. Vi or emacs would clear the error message away. Of course, in a multi-window situation, the compiler output is usually in one window or pane and the editor in another, so this is moot now.
I got a local login on a machine so busted that I couldn't mount /usr, meaning I couldn't read /usr/share/terminfo, meaning no screen editors would work. I knew just enough ed that I was able to fix /etc/fstab and bring the system up.
I wouldn't wanna use it every day, but it has its time and place.
"busybox vi" is another option in these situations when that's available. It will happily run in a blank chroot.
Everybody should know a little ed though, it's not that hard.
It was on a server that didn't have busybox installed. Also, can busybox vi work without access to termcap or terminfo?
Honestly at that point, unless your fstab is really big, I'd just `cat /etc/fstab` and then `cat >/etc/fstab <<EOF` and very carefully retype it (I mean take a backup first of course).