Yes. A man had his finger ripped off by a thief stealing his iPad. It was not for the touch ID, but that is beside the point IMO. There have been plenty of incidents where muggers remove fingers to steal rings. I don't think a phone is so different.

Having looked up this iPad theft, they didn't rip his finger off - the drawstring of his bag got tightly wrapped around his little finger and the force of their grabbing the bag stripped it. An awful thing to happen, sure, but not at all "had his finger ripped off by a thief" - it was a pure accident.