As we (Concrete5 Denmark) are building a lot of websites, we always thought that a clean new site is a good site. Now with extreme Clean we can accomplish just that. All old page versions, all the old log data that shows a lot of errors because we were building and all test users that is not used any more.
But especially the page versions was a concern. We just upgraded a site and at the same time we changed the theme. The site had 12 languages and by that we had a lot of old page versions (about 9000). With Extreme Clean we were able to clean all that old data - Very Cool.