I'm primarily a windows user, but I learnt vim so that I could more easily edit files on linux servers, since it's pretty much guaranteed to be there on any box I'm on.

I don't particularly enjoy programming in vi, but I tend to do it quite a bit when making small changes to our codebase on my dev box in the sky and I don't want to deal with the overhead of starting an IDE locally.

It also stopped people from laughing at me when I ran nano. :p