You have misunderstood what Airport Services do. They are not a way to disable GSX services. In fact, Airport Services don't have anything to do with GSX services: they only affect *default* Ground Services.
Now, I can understand it might seems confusing, because GSX has replaced their models with its own models, so default ground services will now *look* like GSX vehicles ( because they are ), but as long you don't call GSX with its menu, they are still under control of the default Ground Service routines in the sim, the one you call from the ATC menu.
Basically, if you never open the GSX menu and never use any of its features, GSX will act like other "ground replacement models" products out there, that simply replace their models with better or more varied models/textures, but will still work as the default models.
The reason why we supply thousands of Airport Services files with GSX, is to overcome the limitation to be customized "by region" ( the world is divided in 25 ground traffic regions ) but, with Airport services files, we can be more ACCURATE to assign certain vehicles to certain airport, since they are now assigned "by airport", because we have an Airport Service file for that airport.
The reason why we allow to disable GSX Airport Service files for individual airports, is because some developers might have included their own Airport Service files, with custom vehicles and textures that might be even more specific for that airport so, if you happen to have such sceneries, you might want to disable the Airport Service file for that airport from GSX, so the one that came with the airport will be used.
If you are not using such airports (those that comes with their own custom Airport Service files), by disabling the GSX Airport Service file, the only thing you'll achieved, is making the vehicle assignment for that airport to be LESS accurate, since it will revert to have vehicles assigned by its region, instead by the individual Airport Service file.
Again, this ONLY matters when you are not really using GSX, and affects ONLY default Ground Vehicles ( or those used by AI ) that appear automatically on airports BEFORE you call GSX yourself.
Because when you all GSX from its menu, the default Ground Vehicles will be replaced by GSX ones, chosen according to GSX own rules and working under GSX control so, nothing you did to the Airport Services will matter anymore, once GSX started.