wasn't that one of the major revealings the fact that they were not hunt and killing mistings, they were hunting them and using them for spikes. Camon died the way he did because he did misuse allomancy but, he wasnt a misting.
With mistings being so rare wouldnt they need everyone they could get their hands on, not just when we happen to need a spike?
Also, TLR knew of all 14 metals. he tells them how to make em in the caverns. (16 total metals, 2 still unknown)