I have never heard in my life of a car catching fire while driving, what are you running, 3000+ Horsepower?
?
That fire extinguisher definatelly makes NO SENSE at all to me. 
Don't think you'll ever need one?
The wiring harnesses in 90% of these cars are original. Back then, they didn't have the proper materials to make the correct connectors for the fuseboxes and bulkhead connectors so the current running through them was hotter than supposed to be. That's why when you strip down these cars and look at the wires, there's calcium build-up (looks greenish-blue on the copper) from the years of too much current passing through them, building up heat, creating moisture.
Here's another example...I had my car for about 1 week and didn't know there was a small leak on the back of the valve cover. I sat in idle in a drive though for about 20+minutes, my car was ready to overheat.....well when I finally got it on the road and opened it up a little to cool the motor, smoke was pouring out of my engine compartment like there was a major fire. I pulled over and things settled down, but what happened was the oil had pooled and then spilled all at once on the headers, causing the smoke and maybe a fire if I wasn't lucky enough.
Even with newer parts, my neighbor has a 68 Dart with a new 440 in it and while cruising home from a car show, he lost his brakes. Thank God for being on the highway, but as he rolled to a stop, he noticed flames coming out of the hood.....he had blown a brake line up front and fluid had caught fire from the headers. If he didn't have his fire extinguisher, that car would have been history.
End result, unless you biuld your car from the ground up with new wiring and such, there's a good chance something will go wrong with 30+ year old parts. Sound like overkill I know, but for the 30+ bucks it is to have one in your car, it could save your car, even your own life.
