" just wait untill the fire engine arrives," works fine a functioning urban environment, but don't hold your breath out in the boonies (where I like to spend as much time as possible)....
Once it's properly started, no extinguisher small enough to practically carry in the back is effective enough.
Car fires are not to be underestimated; airbags blowing up, gas pistons blowing apart, burning liquid fuel spills, heck there are even cases where the fire short outed the car and the car started and started moving. I can replace my car easily, so not that willing to take too much risk.