Why don't race cars have alternators? It's not because they don't have headlights! It's because that little chargin' whirler saps up an easy 5hp. The bigger the alternator, the more hp it takes away!
An alternator isn't an on/off device, properly setup it only puts out enough to maintain system voltage. If the load only calls for 20A of charging current then that is all any alternator will put out 40A or 200A. There is an argument to be made that having a stable system voltage provides for a more consistent ignition system and a more consistent run. It would be more accurate to say the more
load there is the more hp it takes to turn the alternator.
That said, the reason drag racers don't run an alternator is more for weight savings. As little as they stress the battery between rounds is what allows them to get away with it. You'll note they charge the battery between each round, or damned well better.
That is the only application I can think of that allows for running without an alternator.
