I think I am lost.
"I turn it on just prior to starting the engine. I turn it off when the engine is shut down." "No reason to leave it on"
Am I missing something?
Just trying to point out that my ignition switch is always on when the engine is running and always off when the engine is not running.
The switch routes power to the starting solenoid, to the alternator field circuit, and to the instrument panel...needed to start or run the engine, and not needed when it is shut down.
The OP seemed to be questioning the oft-stated guidance to keep the switch on at all times with the engine running in order to avoid damaging the alternator. He routinely turns his switch off when underway.
I'm not an expert on this, to be sure, but I know enough to know that my alternator is excited from the battery at start up, and self-excites when running, either of which is a suitable way of exciting the alternator.
My concern is that with the engine running, if the switch is turned off, power to the instrument panel is interrupted and I no longer have low lube oil or high temperature alarms to warn me of a pending or existing unsafe condition. That's the primary reason the switch stays on until the engine is stopped.