I started my career doing Perl CGI stuff, and have used, at various points, C, Python, Tcl, PHP, Erlang, Javascript and Ruby in addition to Java, Docbook, LaTeX, HTML (of course), CSS and various and sundry other things.
I think he's going in the other direction: All he knows is Ruby, yet he wants to extend his editor with additional functionality. (Which is less than useful IMO, because most of the time you don't really want to extend your editor in the sense of programming new things. My .emacs invariably has 40-50 lines, but none of them should be called programming).
I know many langauges, but I enjoy ruby the most. I'm not sure why programming an IDE and running arbitrary ruby programs wouldn't be useful?
I can make a one-liner in ruby to do what I want as fast as I can make a macro, generally. Moving files and directories, find and replace with conditional logic, running cleanup scripts, after-update hooks, generated files. Those are just a couple of things I might use it for, and I'd like it in the IDE so I can just work out of there. The point of programming is that it lets me do anything.
For eg. In Perl there is big momentum behind Padre (http://padre.perlide.org/) and there are similar projects in Python (quick google shows Leo & Pyk).
Emacs has handled them all with aplomb.