It improves the security of my phone in the real world cases that I'm likely to encounter. Like friends, coworkers, family members, and significant others. It

It improves the security of my phone in the real world cases that I'm likely to encounter. Like friends, coworkers, family members, and significant others. It's trivial for them to observe my pin, but lifting a print and creating a fake finger ala Mythbusters or compelling me by force is beyond their resources. So it's a trade off, giving up security against a determined threat for a gain in security against casual threats.