Why does that stop you from writing them in CoffeeScript, especially if it's faster? Because you want contributions back to the projects from JS-only developers?
Re:Rails 3.1 shipping with CoffeeScript
by jashkenas on Wednesday April 13, 2011 @07:13PM
(#2444646)
Yep. I'm a pragmatist at heart. I don't think that either Underscore.js or Backbone.js would have gotten the same sort of adoption they've seen (eg. github.com/popular/watched) if they had been written in CoffeeScript. Perhaps in a year or two it would be different.
That said, I did port Underscore.js to CoffeeScript once, and the resulting compiled JS ran (a little bit) faster than the original version.