Oh there was some rotor inertia drama in the past?

it has a purpose. Im still not sure how much it would do in rc. the reason its a thing at all is that Drives with closed loop control (not an esc) have 3 closed loops running simultaneously and they fight each other for processor time. And the code is sequential not simultaneous so these loops fight each other. Current, velocity and position. ESCs only have 1 loop. so a resistance to change in speed or efficiency in change of speed can be managed by rotor inertia (or mass accelerated if thats simpler). if you know the math for each type of application you can control oscillations or lack of response from the motor. in its simplest form ..some motors cog at low rpm. adding the right amount of weight will end the cogging. at higher rpm the cogging still exists and maybe its enough to create some error in the hall effect sensors. there are small differences in acceleration between motor phases that can create small timing errors.

it would be like balancing a tire. but instead of 1 weight at the right spot its a bigger weight on the entire tire. there is more to it then what i explained but that is the fundamental.