Developing Brunch application separately from Rails application is in my opinion idiomatic way of developing web apps:
- Ability to synchronously develop frontend and backend. You can simply use static JSON files on your frontend (served from
public/
) before Rails app is ready. - Ability to make frontend totally independent from backend. There can be many front ends, after all (iOS, for example).
- Ability to share frontend code as an example implementation of your API-using-app. In many applications (yeah not all) frontend these days is less important than the backend data itself and backend business logic. I see no reason not to provide third-party developers a convenient example app they can base they code on.