If support will be added to any of languages such as: CoffeeScript, TypeScript, GorillaScript, LiveScript and any other. It will lead to user segregation and separation of knowledge, as not everybody know them. So users won't be able to learn others code when it is in different languages. Debugging is hard unless you do source maps. Then it will be extra dependency support. And many other less obvious negatives on the community as a whole.
What are benefits? They are very subjective, and opinion based