It also breaks in interesting way: I discovered just today that constants defined on the Model subclass are not available when you use 'get_model()'. I suspect methods wouldn't be accessible either?
If you would rely on code outside of said migration, you would be breaching that frozen state and potentially end up with unintended side-effects (e.g. running a migration created 2 years ago that imports your code that changed today). This is why you might have to sometimes copy-paste logic to your python migrations, but you also guarantee that the migration always runs the same way.