[Leynos]

From what I understand, it's used as a scriptable editor in UNIX automation. It may not be used that often, but it's something I think people expect to be there when a particular piece of behaviour is desired. (Not that I've ever used it myself either interactively or scripted).

It also appears to be part of POSIX.

I imagine though that ex would be the more capable alternative, and since vi is also ubiquitous, I guess that makes ex ubiquitous too. So I can see your point.