I don't think you're going to find a single security professional that agrees with your assessment of the situation.
Not one.
But, to me, from a 'security standpoint', it does make sense to limit the use of the passcode.
Say someone was observed using the passcode (because their OtterBox screws up the ability to use their fingerprint) and that device is stolen. Having a Touch enrolled finger at that point is meaningless. Isn't it?
I love my OtterBox case, but I am about to remove the plastic covering the Touch sensor, because so often I can't get into my damned phone!!!
snark
I'm amazed at the lack of cobwebs in this thread. It's kept pretty clean...
/snark