I know a few people whos cars have randomly caught fire due to electrical problems. One of them had a fire extinguisher and calmy eliminated the fire quickly before it did any real damage. The other, who didnt have an extinguisher, panicked and pretty much ran away. He soon had no recognisable car.
Therefore I keep one just in case. It doesnt have to be forn when nyou are upside down in a field. I also know a guy who pulled someone out of a burning car and got quite burned himself trying to put out the woman who he dragged out as she was on fire!!
He did it, and received an awared for it, but if he had an extinguiser in his own car I am sure he could have extinguished the woman (if you know what I mean) quicker and with less damage to himself.
As above though some countries enforce their posession so it cannever be a bad thing.