Diesel engines emit 15-20% more CO2 per unit volume (liter/gallon) of fuel burned because the fuel contains more energy/carbon.
Irrelevant; they emit less CO2 per mile traveled because of the efficiency improvements.
And once the companies stop cheating, the fuel economy of the Diesel just isn't all that much better than a turbo gas engine.
False. This fix does not substantially affect mileage.
Why put up with extra NOx
and particulates (depending on the car you compare to)
Bullshit. Gassers make just as much particulate, but it's of the most hazardous type, which means their particulate emissions are actually worse than diesel. They also emit more HC than diesels, and unburned hydrocarbons are bar none the most harmful automotive emission. Gasoline also has to be refined more than diesel, which means more energy input and more polluting output. Gasoline engines are shit for the environment.
to save such a small amount of CO2?
Diesels emit less of everything but soot (theirs is less harmful) and NOx (worth the trade).
Just get a gas hybrid and do better all around.
You know that battery electrolyte isn't recycled, right? It's just disposed of and then replaced. A small diesel engine won't give the same performance, but that's fine; it will give adequate or even quite good performance these days. It will give superior mileage, and without involving a battery.
Or a plug-in hybrid like the Volt where you can do most of your driving burning no liquid fuel at all?
Plug-in hybrids at least have a reason to exist, unlike non-plug-in hybrids. But I live in the boonies, so I would still have to do most of my driving on liquid fuel. And they are also quite expensive, to boot.
Since I do very little driving I don't give a shit anyway; I bought an old and cheap Audi A8 Quattro and am restoring it to good-enough condition. (Next: AC compressor.) It doesn't get particularly good mileage or have particularly great emissions, but it did cost thousands and thousands of dollars less than a new car, and it is dramatically better to drive than any econo shitbox. With the money I save not buying a Volt, I could buy an S8 (let alone the A8) and drive it everywhere with a lead foot for years. Sorry, environment! I seriously don't go out much, though. My prior car was a 300SD, which was a bit better on mileage and which ran on a more environmentally-friendly fuel. But frankly, it's cheaper to buy a whole new car than to upgrade the turbo and more expensively upgrade the injection pump on that diesel, so that's what I did. German luxobarge forever. I'm two meters tall, I'm over clown cars.