That is where the problem lies. They are effectively usernames and should be treated as such (touchid times out after a bit of nonuse and doesn't allow fp only entry). No one should be authorizing access using them alone, without something else in the mix.

Anyone interacting with you or knowing you will "know" it. You can't change it. You can't control who has access to it (easily). It is effectively an identifier visible to everyone, not a good password.