I know the biometrics suck, but I haven't considered casual muggers cutting fingers off before...
These were not casual muggers, since they resorted to kidnapping, also keep in mind that this was done to steal a very valuable car.
With phones also now also becoming wallets, it's not outlandish to think a thief would cut off a finger.
More serious, organise, full time muggers might, but I would suspect those are more rare (and you're probably in a whole lot of danger with them anyway. missing digits probably isn't your biggest concern)
http://www.3ders.org/articles/20140308-world-first-3d-printe...