I don't know why they would. Not having a knife is like missing an arm. I use it every day for all kinds of things.

It would need to be a big fang to use it as a knife. I'm not sure how strong a knife it would make either. Do you know if there are any Pokémon who shed really big fangs? The only ones that I can think of with really large fangs are Gyarados, and I don't know if they shed them or not. They don't like to talk much.