A thief who doesn't have something as common as a combo scanner/printer (about $70 at Staples, or $30 used, or free if stolen) and a computer, could just turn around and quickly sell the phone to someone who does.
Or not. As you said, most thieves don't give a hoot about breaking into the phone. They just want to resell it before it gets remotely locked down.
The scanner was not meant to be tightly secure. It was meant to be a convenience unlock feature. As you said, it's better than no or simple passcodes, which is pretty common.
Think about corporate provided phones with email access. Imagine someone getting Tim Cook's iPhone and hacking his fingerprint? I bet most corporations will require a passcode along with a fingerprint.