Andrew,

That would not be the case if the side tone is generated
by the headset.


Well, it would diminish battery life, of course.

Also, the microphone is powered by the intercom/radio through its plug.
Interestingly, the new Lightspeed Zulu has an option to power the mic
from the headset battery, since it can be used as a Bluetooth headset
for your phone and you might want to do that with aircraft power of.

--
Thomas Borchert (EDDH)