However, how much will Ruby change over time. If new people are hired (or your company is bought out, or you leave and go somewhere else), are these other people also going to be in the situation where Ruby is more familiar to them than shell?
Shell is everywhere. It's hard to administer a *nix system without dealing with shell (you can be an application programmer without dealing with shell, but not a sysadmin)
along similar lines, it's hard to be a sysadmin and completely ignore vi, you may prefer emacs, but any system you touch is going to have vi, not all systems will have emacs.
not all systems will have Ruby, Python, or Perl but every system will have shell of some sort.