The fingerprint defeats the person who watches over your shoulder as you enter your PIN and then steals your widget, but isn't willing/able to cut off your finger. Yes, you can defeat that person by effective shading, as you should, but this can't hurt, especially because you can configure which keys require which kinds of authentication to activate — I might set my credit to card to only require a 5-digit PIN, while my root key requires a thumbprint and 10-digit PIN.