I recently decided to learn ed, as I felt it couldn't hurt to be familiar with it, and I was surprised to see that it wasn't installed in Arch Linux by default.
Postscript - ex is the version of ed rolled up in vim, which extends the command set of ed. vi, by contrast, handles colon commands by invoking ed externally.
Actually vi and ex came about at the same time. You can see the POSIX standard mentions [1] ex and vi (not vim) together, and you can also refer to [2], under the "First appeared" column.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ex...