While it may be dirty sourced electricity, there are still significant benefits many people don't think about. As (or I suppose now, if) the grid moves towards cleaner electricity sources, the total emissions go down, where the ICE vehicle will always be an ICE vehicle.

Generators are also much more efficient at converting fuel to electricity. They don't have to provide pretty good power output at all RPM's, they are much more fine tuned. There are also emission reduction options that are economical at the scale of a power plant, but not when attaching to millions of cars.