So earlier I mention that I've never seen a roadside fire (and because this thread got pushed back up to the top), I feel like I should share that 2 weeks ago I saw a fire truck parked the opposite direction on I-25 working on a car that was literally just a shell left (DAMN HOT!!). It was off to the side, and luckily was just before the on-ramp I use, so traffic was a breeze! (small victories, as it was down to 1 lane just before this. Don't worry, I'm not heartless... The news said later that night that no one was injured).
Anyway, I tend to agree with @sts7049. It's only useful if it's accessible, and I don't think most people are going to stick it on the A or B-pillar. In addition, when you see a fire like that I saw two weeks ago, an extinguisher wasn't going to do anything...