The extension testing environment is first and foremost intended to test the functionality of extensions that are to be run on wikis that use the stable versions of the MediaWiki software. There are however extensions like the CreatePage extensions, that have been developed outside the Wikimedia Foundation and that would provide a welcome addition to the WMF projects.
In order to provide the right arguments for such an adoption, it makes sense to test extensions on "head" as well. This argument I posed to Bernard, and he has graciously granted my request. Kim has been asked to make it so.
Several people use the testing environment for their purposes. I learned that even Translate is being tested. If you have a need for testing your extension, the environment can be made available to you too.