That's all well and good, if you do your API with Rails.

Assuming this is an actual API, consider a rouge script slamming your API, whoops, your main app is out as well, nobody is signing up for your service.

Assuming this is for backbone/batman/other-generic-clientside-framework, nope. Too much JavaScript makes everything seem sluggish, and I really don't want to reimplement everything twice (once in Rails for conventional browsers and for people who disable JavaScript, second in client-side MVC)