The point is if the servo malfunctions due to low supply voltage (ie its microcontroller
crashes) then it might do anything, including switching the motor's H-bridge
full on. The motor turns to the end point, pulls stall current and overheats...
It might equally well do a sensible thing and just switch off and wait for better power.
Different makes will have different behaviour, and it usually wouldn't matter
as undervoltage is usually a flat battery (unable to sustain high currents for long).
Mains powered motors can suffer from this kind of issue during a brown-out,
since the reduced voltage can lead to stalling at startup (such as in a fridge),
leading to overheating and damage (modern motors should have a thermal
cutout since there is otherwise a real danger of fire).