Whilst you may get electricity from renewable sources, and that is good for you, when you look at the country as a whole, renewable sources make up a small percentage of the total electricity produced.
Why? As the grand-parent post mentioned, it's definitely not applicable everywhere, and transportation is the sector where we are the most closely tied to petroleum as an energy source.
Just decoupling that, even if there aren't any gains for the environment in gas-powered regions, is a win in my book. Once we're decoupled, we can start looking at these problematic power plants, and replacing them with greener alternatives.
Also, in regions like Canada, we're all mostly hydro-powered anyway, so the gain would be immediate. Even then, a gas power plant would probably pollute way less than the equivalent fleet of cars, due to the fact that we're always running our engines cold in the winter, which pollutes a lot more than an engine running at the proper temperature.
The electric cars problems are mostly magnified by American urbanites. Boston is no exception. It might help if zip cars were predominantly EV, although my urbite friends have mostly replaced Zip with Uber