Imagine the confusion when you're working with or contributing to two different libraries that have different indentation or documentation standards. Now make the language different, and mix and match different syntaxes for different languages. Should modules in JS accept patches in coffee? What about vice versa?
Not to mention that coffeescript then becomes an additional dependency, and while in itself that's not bad, I still consider less unnecessary dependencies a positive aspect to a library/framework.
Edit: Why downvoted? Is this not a valid concern?
Disclaimer - I'm still in the pub, What I'm trying to say is, I understand JS, I'm still happy to code in js, but I like teh coffeescript. Let's learn to speak to each other
To reiterate, I actually love coffeescript, but I see the fragmentation as overall hurting the JS developer community.