It's about the smog, AC
No-one is seriously bothered about the overall emissions. If they were, then "buying a new car" would be WAY off the list, and if a car were needed, then "buying an old, efficient, fuel injected car" (like a Mk II Golf GTI) without a catalytic convertor - and maintaining it perfectly - would be a far better option.
However,
Plug in hybrids allow the emissions within cities to be reduced and they pander to people's car usage habits; most people I know drive around 10-20 miles to work and back again and rarely exceed that area of travel (living in the Scottish Borders gave me a skewed idea of distance - 20 miles to the next town - I keep getting places in the Midlands far too early because it's all so close). They eliminate one of the worst traits of modern emissions controls - dreadful fuel economy when cold. The engine need never fire up for the bulk of the car's usage; when it does it'll be when the car is driving on the freeway, where it should warm up quickly (since the load is predictable, to charge the batteries, it may be possible to drastically reduce the emissions by tuning the engine to run very lean in a narrow, optimal rev range) and isn't contributing to the local emissions that present a problem for America's cities.
As an example of the effect a catalytic convertor has on a car - I had a 1993 Volvo 480 with instant readout MPG and a cat, and an older example of the same car. The cat equipped one, on my drive from the village to the town I worked in (3 miles in total, but not ideal for cycling), would return 10mpg until the cat was up to temperature. Which it didn't get to on that drive. The non-cat model had come off the "fast idle" setting after the first mile and have 30mpg.
Now I live somewhere with public transport, I really try very hard to leave my car alone unless it's needed, although it would be far more convenient to drive it to places than spending £2 on a train and another £1.50 on a bus for some more convoluted routes. If I had a plug-in hybrid, I'd be inclined to use it.
Anyone who has visited America knows most cities and towns are not designed for walking. They're designed for car use. The banks are drive-thru. Without massive infrastructure changes, plug in hybrids are the next best thing for short term reductions in pollution, and in states where energy production is "clean" could have a small impact on the environment as a whole.
Of course, making a car like this tends to involve a much greater impact on the environment than a conventional new car, and it may have a shorter useful life given that when the batteries fail it may well have depreciated to the stage to make economic replacement unviable, but it's a brave step - it's a bigger step forward than Toyota made with the Prius. The main failing is that it won't be on sale for some time yet - GM's shouting about new models seems to get earlier and earlier - can you actually buy a Camaro yet?
