If you want to contribute to the Drupal project, you need to know how to use CVS. It's a version control system, like Subversion, but older and clunkier. Still, Drupal have been using it for *years* so I can hardly blame them for not switching. From the command line in Linux it's straightforward enough, and anyway, I can't talk because they're not switching from CVS for the same reason I'm not switching from Windows (even though it would make far more sense for me to abandon Microsoft and install Ubuntu). In a word: hassle!