I can think of a couple of ways fingerprint scanners could be potentially problematic (aside from, of course, being a way the NSA can automatically get the fingerprints of anyone it doesn't have yet, which is problematic enough).
Bad possibility #1: It can be used to justify legal liability for all actions that take place on the device, since there would be incontrovertible proof that you activated the device (because no one else can).
Example: one could, for example, ask to borrow your device to look something up. They could then use your device to look up something illegal / controversial / etc., and because we all know that everything done on your device is tracked/monitored, you're now automatically put on some watch list because data mining software linked those searches with whatever behavior the government at that time deems inappropriate.
When you are hauled into the secret court (since indictments under recent legislation can be confidential), you will not be able to claim that someone snooped / cracked your password, because there is proof that it was you that activated the device prior to that, and your guilt for the infraction you didn't even know took place is assured.
Bad possibility #2: Criminals who want to steal your devices now have incentive to abduct you, hold you hostage (even if only for a few moments at gunpoint, which obviously increases your danger), because they will need to force you to disable fingerprint protection on your device. In short, it will reduce device theft, but increase the potential danger in situations where determined criminals want your devices. From the point of view of your personal safety, it's better your device is just snatched, as opposed to you having to interact for a longer period of time with the attacker, which leads to possibility of escalation of tensions.