If someone steals your password, you can change it. But if someone steals your thumbprint, you can't get a new thumb. The failure modes are very different.
Bruce SchneierMicrosoft made a big deal about Windows NT getting a C2 security rating. They were much less forthcoming with the fact that this rating only applied if the computer was not attached to a network and had no network card, and had its floppy drive epoxied shut, and was running on a Compaq 386. Solaris's C2 rating was just as silly.
Bruce Schneier