vi is the lowest common denominator among *nix text editors. It's a huge advantage to know how to use it.
But almost any system is going to have ed installed.
As an intern working for Nortel we had HPUX machines, and vi was frequently used. These days at my current job it's an all-Windows stack, so I'm in VS all the time and don't really have a need for vi.
It doesn't mean I don't miss it, though.
It's a pretty impressive emulation, even has some vim features. That package was the only thing keeping me sane during a year spent in VS.
It both has lots of features other editor's don't have, and implements most features differently, so it's not like it's a subset of any other editor.
Vi is only the lowest common denominator of the vi-clones family of editors (Vim et al).