I once placed a shaped counterweight upon my Concept 30 RC Heli, just to show it can be done ... didnt take pics however.
It flew well, though cyclic rate was reduced slightly. "felt slightly mushy" I increased the radios rates to comphensate, and had no issues with it, I believe a web site is still out there who sells these counterweights for the model heli crowd.
It was more fun to demo the airflow around the heli by hovering next to a steam vent in colder weather anyways
I would agree with Lu, no practical purpose, more of a because it can be done type of thing.