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?
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.