This may be half baked but I think there is an advanced scenerio where a package could install additional content files.
Think of this scenerio.
A package is installed in an application that provides a generic start up code. Any class that implements IStartUp is executed from a generic IoC handler.
A new package is added to a project and if this package detects that the Startup package was already applied to the project it adds an implementation of the IStartUp class to register the packages startup code.
Thoughts on this type of scenerio