A "rare" item is not rare if it can be infinitely duplicated. The only point of something being a rare drop is to control the supply, or limit it/make it a reward for effort. Since a teachable recipe can be infinitely duplicated, then you basically remove that content from the game system once a community-minded person gets ahold of it. Even if you argue some people will sell it, you are then providing a 1 time boost of gold to the first person who gets it, then making the content pointless/gone for everyone else, since they'll just go to their local crafters fair to get it. I feel that it's functionally no different than saying "The first person to kill this boss gets 100k gold". It provides a one-time boost of content to an individual or group, but then effectively wastes that development time because the content isn't there for the future. It just becomes another "tax" you have to pay to the incumbent players as a method of redistributing wealth.