>However, a lot of people I talk to are moving to pure JS front-end + REST backend. In this world Rails doesn't deliver any particular value (in my opinion).

I agree with this but I think there is a place for a javascript framework tied fairly closely to Rails. Possibly ember.js since the attitude towards both seems similarly opinionated and Yehuda Katz is one of the ember team. The Rails API gem accommodates this quite nicely as well although one of the benefits of Rails' modular design is that you can lose lots of the stuff you don't need if you are only building a REST API. Maybe this is the future of Rails?