To someone new to the command-line, Vim is not at all approachable. Once you know your way around Vim, it's a fantastic editor, and frankly Nano just doesn't compare, but you can't expect a newbie to be able to use Vim effectively. Without a tutorial and a deliberate effort to learn it, Vim is rather baffling.
To the question of what should be the default, I suppose it depends on the context. A power-user distro wouldn't have much use for Nano, the choice would be between Vim and Emacs.