Two points in favor of JavaScript:
Every CoffeeScript developer knows JavaScript. The inverse is not true. Because CoffeeScript emits JavaScript, and any debugging you do in the browser is in JavaScript, by definition CoffeeScript developers must have at least a passing familiarity with JavaScript.
CoffeeScript jeopardizes the ability to stay up-to-date with the web platform. Parts of CoffeeScript require new syntax that is incompatible with features coming in ES6 and ES7. By using CoffeeScript, you are locking yourself into always using ES5. One of the principles of TC39, 1JS, means that if you author ES4 code today, it will guarantee to continue working in the future, and be easy to upgrade when new features start landing in browsers.
Edit: ES5, not ES4.
