On full scale planes they do just what the name says, it is used to place weight opposite the control surface for balance. This isn't very practical for RC planes, because most of us don't want the extra lead weight in every surface. Also when you mass balance a surface you usually have to build a stronger surface to support the added weight. This ends up making planes heavy.

In one respect they do decrease effectivness of the control surface. However, for 3D flight this starts to work the other way. The bigger the counter balance, the smaller the stab which is good for tumbling, etc, because you don't have the larger flat portion of the tail plowing through the air.

That's my opinion.

Bill