Many people who regularly use both vi and Emacs tend to use them for different things, and find it valuable to know both.

In general, vi is best for small jobs — quick replies to mail, simple tweaks to system configuration, and the like. It is especially useful when you're using a new system (or a remote one over a network) and don't have your Emacs customization files handy.

Emacs comes into its own for extended editing sessions in which you have to handle complex tasks, modify multiple files, and use results from other programs during the session. For programmers using X on their console (which is typical on modern Unixes), it's normal to start up Emacs shortly after login time in a large window and leave it running forever, possibly visiting dozens of files and even running programs in multiple Emacs subwindows.