I think it's worth carrying one simply because if you don't and your car burns out, you'll regret not having one to at least try.
But if it's electrical, it won't stop until the short breaks and by then, something else will be on fire.
If it's fuel - get out and run.
Normally you can squirt the extinguisher under a hardly open bonnet edge, but open the bonnet and it will go WHOOSH!
Morgan louvres though......... no chance - it will get all the oxygen it needs.
I've seen two car fires. The first one was local and it was me who rang 999 from a call box - long time ago. I then drove back to watch. They lifted his bonnet and dumped a massive amount of foam which enveloped half the car. And amazingly put it out with minimal external damage.
The second was at Torquay - a Chevette. It was well alight when the brigade arrived, and the fuel pipes had melted, so the fire was running towards the tank. Despite several minutes, it kept relighting with the heat, so although they kept the tank cool, the rest of that car was burnt out.
The thing is, have a fire and you won't be driving home that day, unless you're really lucky.
