Tuckunder,
That is a question that is best answered by the designer, or a qualified engineer who becomes familiar with the design. My first guess is that the extra blade weight might actually subtract weight from the MAUM, since the stresses for the greater centrifugal forces will impact the rotor head, and could make the head have less structural margin.
If the head is grown in size and strength with the blades, your logic could very well be correct, however, as long as the rotor blades do not add big changes in the bending stresses they impart on the mast and transmission. If the head is a teetering, "semi-rigid" head, the bending is the same, if the head is articulated, the bending can go up a bunch.