The problem with cars like that is all you are doing is moving where it gets it's power from. So by not putting gas in it you are charging it over night using depending on where you live "dirty" power.
At least by centralizing where the "power" is produced you can improve the clean efficiency through economies of scale in the short term and by replacing dirty plants with clean in the long term. Or would you rather just keep millions of ICE's on the road?
As for the Tesla S, it's really big advantage is charge time. They are claiming to be able to get %80 charge in like 20 minutes. If true, that's awesome. The problem with electrics has always been that you can't just "gas up" real quick for extend journeys. Nobody wants to wait all night just to recharge every 300 miles. But taking a 20 minute break every 240 miles? suddenly electric becomes viable for long distance. Now if they can just talk a line of gas stations to add recharge points - bam! wide spread adoption.
20 minutes is still not as fast as getting a tank full of gas, But it's close enough to get electrics from major inconvenience to minor inconvenience.