I don't think this is a one shoe fits all type solution that some people say it is.
Reducing voltage generally will reduce the power, but potentially produces other effects which may offset an gains (heaters may need to run longer, light levels may decrease requiring more fittings, torque will reduce on motors etc.). I guess it could be useful if applied in selective situation and where the voltage is for some reason to high for the design condition. You would also need to ensure that any device does not result in the voltage dropping below the minimum imposed by manufacturing standards.
To me, designing the electrical system itself to deliver an optimized performance would be the best way forward. Adding a device to cater for what is perceived as an electrical system design shortfall, is not something I intuitively like.