i believe that the counterweights on an inline6 crankshaft are there to counteract the reciprocating mass of their corresponding connecting rods (the center-of-mass of a con-rod goes through a circular motion as it reciprocates).

however, the "self-balanced" nature of the inline6 has to do with more with the movement of the pistons balancing each other out, in terms of forces and moments. No additional counterbalance is required to compensate for this.