Someone unfamiliar might just be dumped into vi because some other program thought it'd be great to open a text editor, e.g. for a commit message. You're unlikely to run vi intentionally unless you also know you want to run vi and how to exit, I guess.
Actually, defaulting from $VISUAL to vi and from $EDITOR to ex (or ed) is what -- strictly speaking -- makes sense.
Remember what the difference between $VISUAL and $EDITOR was intended to be. There's a whole range of places in Unix where there was, and even still is, a distinction between a line editor and a full screen editor. Consider, for just one example, the ~v and ~e commands in BSD Mail.
