Yeah, and if you're being robbed they'll probably fuck you up pretty badly if you don't. While not mind readers, they don't have to be.

In most cases anyone who wants to beat your pin will just watch you unlock your phone. They don't need to see the exact numbers: you can pick most of it up just based on relative finger motion. Looking at screen smudges gives another set of hints that are pretty trivial to extract.

If your goal is to keep it from the police, your best bet is to use an entirely different system than rely on your phones lock screen.

I can't think of a scenario where I'm thinking both "biometrics are insufficient to safeguard this data" and "it's appropriate to keep this data on my telephone".