To be honest I'm not all that familiar with Redmine's use of plugins, but other "pre-existing" OSS Rails applications certainly do make use of plugins in a number of ways. Rick Olson's Mephisto blogging package leverages plugins to handle extensions to the core functionality, for instance.
Other applications, such as Radiant (CMS) and Spree (e-commerce) have their own extensions architecture that differs from plugins, and makes it easier to extend them in ways that make sense to their domain. In Radiant, for example, the extensions system allows you to easily add new Radius tags, page elements, and administrative sections to manage custom models.
posted 11 years ago
Also, you may want to check out this link to the Redmine wiki: