I don't know that this answer is in the spirit of the question you're asking, but I think anyone who does software development for a living needs to know at lea

I don't know that this answer is in the spirit of the question you're asking, but I think anyone who does software development for a living needs to know at least the basics of vi(m). If it hasn't happened yet, some day you're going to find yourself logged in to an unfamiliar Unix-y system's console and need to edit some files. You may not be able to start X and get to your favorite GUI editor. Emacs may not be installed and you may not have the authority or time to get it installed. (Side question: Do any Linux distros install Emacs out of the box?) But vi(m)? Yes, it will almost certainly be installed, and you're going to want to know how to use it then.