There's 1 company theme but many different web applications with different theming approaches, for e.g. mailing list software, issue tracking software, CMS, blogging s/w, etc. The company may wish to use for e.g. WordPress to manage their blog rather than whatever comes native with their CMS. Management may decide for e.g. to move to a new CMS platform. Upgrades do tend to also mean rewriting applications to work on the new version and this can be a long drawn out process. And usually by the time you've completed it the new version of the software has been released.
ContentMirror - Manage Your Web Content In Plone and Serve It Up In <Insert Favourite Server> Tim Knapp