As a result, we've seen apps like rdio using the launcher in a way that goes against the user's expectations of how a launcher should work. Instead of zooming the controller in from the center of the screen, the controller gets pushed onto a navigation stack. In rdio's case, they should have built a simple table view with each 'sub-application' having its own row. Tapping one of those rows would then push the controller onto the navigation stack in the same way that it currently does with the launcher.
-- From a holistic user experience applications "spring forth" from the system launcher, creating a consistent feeling of two functional modes in the operating system. The first being finding and launching applications, the second being using the launched application. Introducing a third layer of launched sub-applications complicates that model, oftentimes unnecessarily.