Marginality & low-inertia
Gouli & Lonewolf,
Having lightweight rotors - which is effectively all low-inertia means, give or take some ability to pull some mass inwards through clever composite structures - isn't necessarily a bad thing.
It does reduce the amount of energy stored in the rotor, potentially allowing it to winddown more quickly.
On the other hand, it saves fuel by having of order 100kg less to carry around, allows a quicker acceleration of the rotor with changes in power/airflow at low pitch should it get slow, makes debris a little less dangerous in the event of it coming into contact with something, and provides less gyroscopic restriction on the motion of the aircraft. By reducing the rotor mass, all of the bearings and shafts can also be made lighter, making the whole system more efficient. When there's not a huge overabundance of power available, as you say every 50kg counts.