Personally Coffeescript is ok, but I would rather it be a supporting player in my Rails app, not most of the logic. From what I can see, you still have to duplicate a lot of the logic when using Backbone.js or Spine.js in Rails. Can I not get the benefits of these frameworks or make what is essentially a single page app with js for real time updates using just rack-pjax?
Well what do you want to achieve? No more page refreshes? In that case rack-pjax should work. If you want more speed or a responsive UI then I'm afraid it won't be a good solution.
A single page app is preferred imo because it seriously decreases the load and complexity on the server + there is a nice abstraction between viewlogic & serverlogic.
Your server would basically be an API and your client would render all of the API data to the browser. This way the server is heavily simplified and has a lot less work to do. (win!)
On the client side we can also see a lot of improvements. If done right it rerenders itself continuously based on events and statechanges made to the data. This approach leads to much less coupling (and duplication) in your UI layer and a more responsive UI for the users. (win!)
If you don't care much about that then go ahead and use pjax :)
Keep in mind though that rewriting an existing server-side-view-rendering app to a single page app using a JS frontend is tricky business. It'll probably end up in a major rewrite. You could also experiment with writing just parts of your page in a JS frontend.