I don't know if this has changed but Gentoo, the distribution I run, is very nano-centric and doesn't include vi on its installation disk - at least not since last I checked. It is an odd exception to the rule of vi being available everywhere (which it should be).
I got used to nano, which is easy to use but has less features than vi, and then I got into this environment at work where I had to telnet to an internal HP-UX machine, and then from there to a system on another network segment which ran some old version of Solaris. Each of those machines *only* had vi installed (well, they didn't have pico, nano, or emacs - maybe they had something older). I was forced to learn vi, which was infuriating at first, but it goes to show that, as several have pointed out, it is ubiquitous, or nearly so. vi is something that anyone can learn, but I can't say it is intuitive or user-friendly.
It's just that, if you do learn it, you can do a lot more with it than you can with nano. The time investment up front might be worth it. And you really don't want to ever encounter it, and have it be your only choice, in an emergency, having never used it before. This is how I first encountered it (in terms of having no choice but to use it), and it was not pleasant.
Me, I think the BANK STREET WRITER should be the default text editor on all operating systems.
Because I am all about the danger.