Developing our first open source Drupal product, Watershed now, as well as in our recent work on internationalization in Drupal 7 with Meedan.net, we've run into some bottlenecks managing the testing of new releases of Drupal installation profiles and features. The only way to make sure that installation profiles are built correctly obviously is to build them up from scratch. Features exports are great, but they too can be a little tricky, and thus require continual testing.
Especially for complex Drupal distributions, running an installation profile by hand takes a nontrivial amount of time. Then, clicking through and testing that everything is working correctly is even more time-consuming. Wouldn't it be great if there was a way that you could just check your new Drupal code into version control and have a dozen helpful little gnomes do all this testing for you?