Feathering the blades is windmill "gearing". It matches the power available from the wind to the power that can be absorbed by the generator. In the overpowered case, it scales back the power converted from the wind: it "spills" some of the wind energy by making the blades less efficient so that less power is generated (totally feathered results in no power, but the blades can be feathered to any degree necessary).
Adjustable gearing would add a lot of complexity, weight, and reduce reliability. Reducing efficiency by (partially) feathering the blades is a much better method, especially since it is already a necessary capability of the windmill.
Adjustable gearing would add a lot of complexity, weight, and reduce reliability. Reducing efficiency by (partially) feathering the blades is a much better method, especially since it is already a necessary capability of the windmill.