As I said, even if he doesn't want to install emacs (not that there's any actual, let alone good, reason to do so save being an asswipe) there is no reason not to install mg.
 unless you refuse to install any other editor than `ed` and `sed` on servers because you believe no file should ever have to be edited there. That's perfectly reasonable. But then I won't find myself having to edit a file on a server in the first place.
I use emacs... all the time. It's my favorite editor, bar none.
I use vi/vim every day, because it's always there.
It's not that no file should ever be edited there, but, especially in today's clustered world, it's good practice to keep your servers as minimal as possible.
It might sound silly, but keeping to principles and controlling as strictly as you can get away with the changes that go into your production servers is a good practice that tends to keep things working.
vi is handy because it's always there. Emacs is fine - but it only REALLY shines for me when I have my custom environment set up - for a quick edit, it's just a quick edit.
Also, the 'vi' on most unix/unix-like systems aside from Linux these days is nvi, not vim. which is about 200K, not 2G.
And you completely miss the point of mg:
> mg is intended to be a small, fast, and portable editor for people who can't (or don't want to) run emacs for one reason or another, or are not familiar with the vi(1) editor. It is compatible with emacs because there shouldn't be any reason to learn more editor types than emacs or vi(1).
> mg is just a text editor with Emacs-like bindings which is not worth much to me.
It is of worth to me when I need to edit files on an other machine than my own, I've got the basic navigation and edition bindings embedded in my nervous system (even more so as they're also the edition shortcuts of standard Cocoa controls)
> Also, the 'vi' on most unix/unix-like systems aside from Linux these days is nvi, not vim. which is about 200K, not 2G.
I said 2M not 2G. And OSX uses vim as well, so most active VIs are probably vim not nvi. But even assuming it's nvi you have, that merely makes it as small as mg, still no justification to not have mg.
 I'm not sure any non-BSD unix-like uses nvi actually, MINIX has switched to elvis and OpenSolaris looks like it uses vim.