CNC, you are correct but you are only talking about the mechanical balance of the rotating assembly. Or to put it another way, balance effects that you might feel outside the engine. If these were the only concern there would be no need for counterweights at all on an inline six.
Counterweights reduce crankshaft bending stresses. I suspect this is the primary reason GM went from 6 to 12 weights on the 292 cranks. Nothing to do with mechanical balance but major effect on bearing loads and crankshaft durability. It is also likely that 6 large weights versus 12 smaller weights will exhibit different torsional vibration characteristics.
