Play [1.2.5] Newbie question on using Play UI templates Vs other UI frameworks

[gokool]

I am fairly new to Play framework. I tried the "yabe" tutorial on the website and am really liking Play for its simplicity esp. with JPA/Hibernate.

Next, I have started playing with the UI templates and frameworks for front end. Although Play's templates are easy to learn, I am reading on blogposts how most folks are using a Javascript framework like Backbone or Ember and leveraging Play for only the Model and Controller functionality. Is this done only when you are building a rich-UI web application like Gmail which mostly are just one or two pages or is this applicable for building any kind of web-applications.
If I am just building a Gmail like application with JS-framework on the frontend and a Service Facade at the backend, isnt using more lightweight like JAX-RS be a better way.
Sorry if this is a basic question, but just trying to wrap my understanding on where Play fits in.

Thanks