Drooping RRPM
We have to remember that everything in a helicopter is a compromise.between the main rotor,tail rotor, stabiliser, type of head, low speed ,high speed, etc. We will never be at the best efficiency all the time. As Nick and Dave have pointed out it is a balance.Another important point is control response. Generally at high Nr you get a sharper response. Close to the ground that makes me happier. Away from the ground reaction response is not usually that critical, so we could operate at reduced Nr if desired, but lets keep things simple.If you operate with lower Nr you must use a higher blade pitch angle
If you look at a fuel flow diagram for weight /altitude you can see the fuel consumption decreasing up to about 5,000ft. This demonstrates the increased efficiency of the rotor at the higher blade pitch angles required to induce the flow up to the point when the fuel flow starts to increase. This must be the peak point of rotor efficiency as above this higher blade pitch angles mean more rotor drag, hence more fluel flow.The gas turbine engine,meanwhile, will maintain its power output from MSL with improving SFC as a measure of its efficiency as altitude increases until it reaches the design Erpm.
Overall, the designers strike a nice balance.