[rdl]

I do, but typing in a long passphrase every single time you unlock your phone kind of sucks; if I had a 4 digit passcode I might set a shorter relock interval.

I'm not so afraid of someone's stealing my phone, then coming back and cutting off my thumb. If I were using the phone, it'd be easier to come up at gunpoint and grab the phone while it's unlocked, if you're that paranoid (one of the reasons highly sensitive data isn't unlocked "in the wild" in sensitive organizations).

Simple theft or losing the phone is still the most likely, and a biometric+PIN, securely stored on device, solves this.

High-end luxury cars have great engine immobilizer systems, which led to a lot of carjackings, since it was easier than unattended theft, which is basically the problem you've identified.

There are LOTS of other issues with biometrics, but they mainly come up when they're part of a centralized service and can't be completely controlled by the user.