… First, a user must enter a passcode to initiate the iPhone 5s' biometric capabilities. As a protection against nefarious users trying to bypass the security measure, only this code can be used to unlock a device after it's been rebooted or has been left unlocked for more than 48 hours …
This is such an obvious flaw. It will almost certainly generate huge amounts of user frustration and many complaints.
The type of person they are intending to use the sensor are those that normally don't use authentication in the first place. So these folks (and I know them well) will put in a passcode to start using the sensor and then immediately forget what it is.
At some point they will leave the phone alone for too long and get locked out. Then the only way back in is to remember that password they put in months or possibly years ago and never had to re-enter. It's a virtual certainty that they won't remember this passcode, and there won't be anything for it but a trip to the Apple store.