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Well that's how they do it?
I know that the older Permanent Magnet Alternators were regulated by sending excess power to ground. that means the extra current that would allow voltage to run too high is sent via a regulated transistor to ground. That's a controlled resistance, so heat is released and you need a heat sink. Motorcycles use this system.
Old English bikes like my BSA just used a large Zenir Diode that would spill all excess power to the heat sink above the 14.5 nominal volts that it was rated.
I heard that newer PM alternators regulated differently so they didn't 'waste power', switching could do it and thus reduce the heat of that wasted power.
Standard automotive alternators (Trucks boats airplanes) have non permanent electro-magnets in the rotating assembly. The regulator controls current thru slip rings to the electro magnet. This is called Field Current because it adjusts the magnetic field strength so the alternator voltage output is maintained at the nominal voltage (14.25 to 14.5 give or take) So the alternator only takes as much power from the engine as required to maintain the voltage up to the max current output of the alternator. If this alternator were turned off, it would just be a belt tensioner.
For this reason, I would think the permanent magnet altrnator would be used all the time at capacity, and it would be sized to meet the minimum power requirement of the aircraft. The Field Current controlled alternator would carry all excess loads like recharging the start battery, and auxilliary equipment, say the full avionics suite and entertainment air conditioning what have you.
Scott
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