I can understand FTD's point of view. The engine is what's most likely to have a fire and by the time you detect it you may just want to let it burn! This actually happened to someone I know who will go nameless here. His 911 started to burn and by the time he had smoke pouring out of the vents it was pretty bad. He let it burn a bit before extinguishing it just to make sure it was gone. Sometimes it's really better to total the car than to try and repair it. This guy now drives a nice 911TT.
I'm not sure it's really insurance fraud if you hesitate to interceed and mitigate any damage, but obviously this is a bit of a moral dilema still.
With the Boxster an engine fire would be even tougher as there's no good way to get access to the engine, ESPECIALLY if the car is on FIRE!!! Still I'm in the market for an extinguisher too. I've got one mounted in my crappy Honda, why not my Porsche then? If nothing else you might want to help someone else with a fire in their car!