This is a way to get normal people to use better security than nothing and to give them the convenience of not having to enter an App Store password every time they install an app.
It's not meant to protect special forces operatives in the field or CIA analysts' contact lists.
Having said that, the idea of losing a finger to access the device doesn't really make much sense. You've got to incapacitate someone pretty well to take their finger off, so you might as well just force them to touch the phone while they are so incapacitated, unless you really like chopping off fingers.
How do you propose it would beyond attempting to measure the amount of capacitance and mapping it to an accepted band? Attempting that is way too fragile a solution due to variability in humans and local weather conditions.
And even if you did put in the effort for that, an attacker could still fairly easily match the dead finger capacitance to the correct band pretty easily.