Apart from all the finger chopping off scenario's, what about situations where you do not want your phone accessed to maintain your rights to privacy?
For example, if you are (wrongly or rightly) stopped by law enforcement who insist that you unlock your phone so they can search the data?
There were reports on MR this year, that Apple were inundated with requests to unlock phones of suspects. Now an officer(s) merely needs to physically place your finger on your device.
I am not advocating helping criminals, however you need to consider that not all people arrested are guilty, or criminals. Furthermore, they have legal rights (in most civilised countries) to keep information they store on their devices private until otherwise legally required to do so.
Is there an off switch? Can you revert to passcode?