Once you are doing AJAX you are going to be writing Javascript (or something that compiles to it). Ember.js can help with that. So rather than writing a bunch of code to pull down JSON from the server and manually update the DOM in ways that deal with various browser quirks you can say "here's my model, here's how I want it rendered, link it to this REST API" and it will take care of a ton of heavy lifting for you.
It's true that it doesn't give you any magical power that you didn't have before and you can do everything it can do with vanilla javascript or any of the other frameworks. However that's a bit like saying you don't see the point of rails because you can just use vanilla PHP for everything.
You're speaking as if doing AJAX and doing client-side MVC are one step apart. Moreover, you're speaking as if using server-side MVC forces the programmer to use no JavaScript at all. At best, this is naive. Either that, or you deliberately misrepresenting technologies to promote something.
The simplest way to use AJAX with a server-side MVC is by fetching blocks of pre-rendered HTML. This is commonly known as AHAH. It's dead-simple to do, powerful and trivial to use via progressive enhancement. That is what you should treat as the server-side alternative to client-side MVC.
If your app has a lot of rich client functionality, it can be a lot easier to just use JSON data with client side MVC rather than having to constantly regenerate everything on the server, especially when several parts of the page need to be updated at once and a lot more performant.
It's true that it doesn't give you any magical power that you didn't have before and you can do everything it can do with vanilla javascript or any of the other frameworks. However that's a bit like saying you don't see the point of rails because you can just use vanilla PHP for everything.