> (like when unlocking the phone or authorizing a payment on the app store)

Aren't these the things where you'd want security?

Unlocking your phone is done so frequently that many people opt not to secure it. The best security is one that you use. It's arguably more secure than a 4 number passcode, which someone can easily watch you enter over your shoulder.

Buying things on the App Store requires you to type in a password for the first purchase every so often. Then you use your finger print.

> It's arguably more secure than a 4 number passcode

My friends often have to share their passcodes with me; at which point it's like, what's this adding?

I don't have a passcode on my phone to protect it from my friends ... similarly I don't have a lock on my front door (that I give spare keys to friends for) to protect me from my friends.

This is a good point.

I guess that I think once you share it a few times and it's used regularly, then it becomes fairly public knowledge. Maybe this is a mis step from password security.

Passwords can always be changed to "reset the access list" if you will, if you're uncomfortable with how widely spread knowledge of the password has become. Biometrics, on the other hand ...