One case for which I’d like to use it would be for the Flex SDK. I want to automatically download the Flex SDK zip as a url dependency (see http://issues.gradle.org/browse/GRADLE-2179) and then I would unpack the zip file to a specific directory from which my plugin can access it. Another case would be for the Flash Player executable, which I also want to automatically download and store it in such a directory.
Somewhere below the root project’s directory could be another good solution, saves me the effort of creating version directories for each unpacked sdk. Only downside could be that it takes up quite some space if you have multiple Flex projects since the Flex SDK is about 230MB. So if you have 5 Flex projects with a different root project, all with the same version of the Flex SDK, then it would use up 1.25GB.