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The centralization of power would be an auxiliary benefit of a plug-in hybrid, but keep in mind that today's hybrids are never actually plugged into the grid - so there's no shuffling of energy costs from your car to your home and no power plant that enters into the discussion yet. The power source that grants an electric vehicle efficiency over a gas engine is the road itself. As the car rolls downhill the electric motor acts as a generator and starts charging the battery "for free".
That's where the efficiency comes in: you can charge a battery using inductive current from the motor, but you can't use that energy to put more gasoline in the tank. In a gasoline vehicle it's just waste energy.