Rowan,
Without two plugs you have no engine, as no oil will get to the rod bearings. The dimples are intended as anti-loosening measures. On your model, the plugs are different from eachother, with different shapes on the inside for centering and stabilizing the sludge-trap tube that fits inside the crankshaft by running through the journals.
Old plugs are removed by carefully, and with a light touch, drilling away the dimples and working out the plug by the screwdriver slot, which ofter doesn't work, or by a variety of other extraction methods. In my opinion, for casual riding, rebalancing just because of the difference in plug shapes seems unnecessary. Others are more expert and may hve different opinions, but, to me, the sludge that will form in the trap represents a greater source of potential imbalance.
Richard L.