[ralphc]

One advantage of vi is that even though emacs is available on all those systems, vi is actually installed on all of those, setting aside windows. If you know rudimentary vi you can walk up to any of those machines and edit a configuration file well enough to work.

[wpm]

mg is installed on macOS by default and is basically diet vanilla emacs. Works fine for quick edits.

[Ferret7446]

vi isn't even in the same neighborhood in this regard. You can and should learn vi (and ed!) in one afternoon even if you never use them. Whereas Emacs isn't something you just learn in an afternoon. (And vim is not the same as vi)

That is a separate discussion from whether you should use ed or vi normally. You learn the "fallback" with the hope of never being forced to used them.