Having been a user of *NIX for over 20 years and a professional Systems Administrator for over 10, I have never once had a reason to use ed. Even the smallest embedded environments usually have some variant of vi available. The only time that I've needed to use ed was edlin in the dark days of early MS-DOS, which is very similar. This is borderline masochistic.
It's not whether 'vi' is available (it usually is), but whether it is behaving properly. Situations do occur where any fullscreen editor will fail and where 'ed' will work as expected.
And at such moments, even though you don't know the full extent of the power of 'ed' you will be happy to know how to change a line and save the config file (and actually quit 'ed'; that's harder than it sounds)
Can you give a specific example? I have a not insignificant amount of experience with thousands of servers under my belt and I have yet to ever need to type "ed" into a server. I did find edlin useful in DOS 4, but that was ancient times.
You narrowly missed the days when it was useful. When I started regularly sysadmining in the late 80s/early 90s (after being a mere user for the a few years) I found I had to learn ed. /usr was always a separate filesystem so any time there was a problem booting you didn't have vi available. If you didn't have at least rudimentary ed skills fixing the simplest problems would be rough.
In linux at least the separate /usr never seemed to get that popular. If you can mount the root filesystem you've got basically the whole OS at your disposal.
I will say that learning ed (or, better yet, ex) well does make you a more efficient vi user. When all you have is command-mode you get very adept at using it. It's actually sort of fun, too. Just hit "Q" in vim some time and see how you do with only ex commands.
I own a Unix introduction book from the early 80s (I think) that describes a customized/extended version of ed as the author's choice of text editor. vi is mentioned as "this newfangled editor that has more features than anyone needs and is such an incredible resource hog that it leaves barely any memory for the actual document on a large PDP-11".
