Author of ember-cli here (and previously the maintainer of ember-rails), this is essentially how my team uses ember-cli at work. Purely as a development harness and deployed to work nicely alongside a rails app.

One of my co-workers has a great related talk: https://speakerdeck.com/lukemelia/lightning-fast-deployment-...

And another related blog post: http://blog.abuiles.com/blog/2014/07/08/lightning-fast-deplo...

I suspect we will want to migrate the ember-rails story to be officially more aligned with the above approaches.