The nice thing about vi is its ubiquity. I can ssh into basically every UNIX system in the world and be reasonably certain that vi will be available. The same can't be said for BBEdit, TextMate, EMacs, Gedit, etc.

I picked up vi because I was stuck doing development over SSH from my college's public computer lab when my PC died. I stayed because once I'd learned the keystrokes, it really was a pretty decent editor. I think that Eclipse/TextMate/Gedit/etc. have some nice points and are definitely better in some respects (like, say, visual presentation), but vi is Good Enough and available basically everywhere.