The model .xml is not always copied, it depends on the situation, if the livery is "just" a livery repaint without a local "model" (or model.xxxx ) folder, and the airline IS recognized from the icao_airline parameter of the aircraft.cfg file, the exterior .XML MUST be copied, because the attachment for the Crew must be different for each livery to support the different crew characters which are associated with the airline.
If the airline is not recognized (or it is, but GSX doesn't have a custom crew for it), the exterior model won't be copied, because the seated passengers and the generic crew has been already added to the exterior .xml of the base model.
If the installer creates a model folder that doesn't exists, it's because something is missing from the aircraft.cfg of the livery, either the icao_airline or the texture or the model folder are either missing or wrong.