Script classes inheriting from application class

I was reading a couple of older threads from 2008, 2010 and wanted to verify that script classes cannot inherit from application defined non-abstract base classes. The question of “why” also came up. I'd like to allow script developers the flexibility to extend/specialize base classes that are implemented in the application interface. I also read some references to registering an interface with AS, but as far as I could tell this would not allow inheritance of base class functionality. Please comment.