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It automatically helps the enviroment if one has the option to purchase emissions-free electricity http://www.energymaine.com/mresources/. If I could afford an electric car that gets its power from my already green home electricity supply, the environmental impact of my family's energy use would drop dramatically.
Even without completely green energy production, though, shifting the energy source from gasoline to power plant can help to make the problem more manageable, even though it certainly does not make it disappear. Point-source pollution is a lot easier (relatively-speaking) to clean up and control than a distributed source like a gigantic fleet of cars.
Upgrade one power plant to a cleaner fuel, install new scrubbers, etc, and you have automatically updated all of the cars that get their power from that plant.