Just as a Smith & Wesson beats a full house, a car thief has proved that a machete can beat a fingerprint-reader anti-theft device: Carjackers swipe biometric Merc, plus owner's finger | The Register:
[I]n most cases there will come a point where you actually want the owner to be able to disable the security quickly and easily. At minimum, biometrically-locked motor vehicles should surely kick up a ('Disable fingerprint security? Y/N') dialogue whenever you stick your finger into them..
It's a horrific example of why many 'security' systems haven't been thought through in terms of 'worst case scenario'. I suspect people are a bit squeamish and would rather ignore the potential for this sort of situation to arise.
Ultimately the Merc's system was a perfect deterrent against having the unattended car stolen; but it created the worst possible problem in a carjack scenario. If the owner had been able to turn the system off, they might have kept that fingertip.
Oddly enough, some people in Hollywood have actually thought this sort of thing through - I've seen several movies where variations on this theme have featured. Other movies are short-sighted though... I think it's one of the Back To The Future movies where the lead is asked to 'thumb some money' - using a device where you simply whack your thumb onto a reader and BAM the money is sent. Even as a kid I remember thinking 'what if you knocked someone out - or even just forced their thumb onto the reader?'.