But if you are targeting a browser and a native iPhone app, the view logic lives on the iPhone app. It used to be that you could just serve up a web page and assume that your client was a browser. Not anymore, and that's my point. And my point is that the trend will continue: I think we will have more variety of non-browser clients in the future, not less. Your car, refrigerator, geranium, etc. will all be hooked up to the web.
Keep in mind that I'm not talking about whether you should create your web app in Backbone.js today, but I'm looking years into the future, anticipating trends.