Rails is just an API
Let's say you have a website and let's say that website serves a lot of traffic. Now let's say you relegate Rails to act as just your API because clients are fast now and you want to do some new cool stuff with Javascript also, you're convinced your site is more like an "application". So you build this system and it works really well, except for one thing. The first time you load the page it takes a second or two minimum to get the page loaded. That's ok at first, but really soon it becomes annoying. So you do everything you can with your client, caching whatever possible. You get the load time down to something like 1s. That's better but still not great. Some static sites can spit back pages in like 200ms  (across the network!!) and that 800ms difference is crucial to the overall feel and snappiness of your "application". Eventually you give up, you keep the parts of the site that work well as a client-side app, push the rendering back to the server because even though the server is a long ways away from the client and even though clients are fast, you can directly control how quickly the server builds a page and you can optimize that to your heart's content. Now you've gotten your page loads way way down to like 400ms. You're doing great, people love you and you've saved the day. I've fabricated the numbers, but this story is based on truth, because you know who you are? Twitter, Alex's employer.That's not to say that Rails doesn't make a great API and that Alex doesn't have a point. There certainly is a place for frontend applications out there built on frameworks like Spine, Backbone or Ember (all great projects). Those types of applications have their advantages in some cases. But, it's prudent to be pragmatic and to recognize that the times where you truly need to build a client driven application are few and for the other times Rails is still great at serving up HTML.