I was concerned about this as well, but I'm successfully using an Ember app as just part of my application. The plan is to use multiple ember apps within the same rails app.
Can you talk a little more about how you're doing this? And have you been successful so far? I'd love to use ember in a few areas of our app while keeping most of the existing structure in rails views, but my (limited) experience with ember is that it looks to be more all or nothing. I'd love to be wrong about that though!
There are better ways to do it, but I use a javascript include tag to render the directory that holds my ember app in the view file of the route I want to render it in.
