I suggest that those who don't believe there's a danger to leaving lithium ion batteries charging seek out and watch the recent NOVA special entitled "In Search of the Perfect Battery". They discuss the dangers of lithium ion batteries with experts and participate in stress testing of the battery packs. Lithium ion batteries are surprisingly energy dense, and it seems that they form dendrites causing in some cases shorting, resulting in sudden discharge with dramatic results. The resulting discharge causes what can only be described as an explosion and fire. I was stunned when I saw the footage.
There is a scientist who has developed a lithium battery immune from this condition the NOVA special shows it being subjected to extreme destructive acts without even heating up. It isn't yet on ready for the market, more testing has to be done, but it shows great promise.
After seeing this special, I would personally not leave a lithium ion battery charging in my car. Yes, NOVA did show a video of an SUV on fire as a result of the owner having left his phone charging in the car's power port.