Depends on the opportunity cost, really. If stopping at the airport adds 15-20 minutes to the trip to downtown, it probably isn't worth it and you want to run

Depends on the opportunity cost, really. If stopping at the airport adds 15-20 minutes to the trip to downtown, it probably isn't worth it and you want to run separate trains. On the other hand, the high-speed train going to Amsterdam Zuid via the shortest possible route literally passes through the airport Schiphol already - a stop only adds 2 minutes or so and is therefore a no-brainer.