Finally, a slashdot topic where I can be informative. Disclaimer: I work in the industry building fingerprint sensors.
Fingerprints aren't perfect security. As so many others have pointed out, you leave them everywhere. That doesn't mean that they're not useful.
1. It's extraordinarily difficult to create a fingerprint spoof from a latent print. Yes, there are people who can do it - I can do it - but it's not easy. Notice on the videos of breaking into the iPhone 5s or 6 that latent prints are taken from a single fingerprint placed carefully on a squeaky clean screen. On your average phone, not so much. Someone who picks up my phone off the seat in a subway will be incapable of breaking in - unless I've just cleaned the screen with windex and carefully placed my fingerprint on it.
2. A fingerprint on a phone makes an excellent two-factor authentication system. The average hacker in east Elbonia can't break fingerprint security - because they don't have my phone or my fingerprint.
Perfect? No, but strong? Yes.