What is the specific advantage of Angular.js as opposed to a view engine like Hogan.js?

The MEAN-stack (MongoDB, Express, Angular.js, Node.js) is frequently portrayed as the new best practice to build Apps. It consists of a Node.js/Express API which communicates with a MongoDB database, and an Angular.js front-end app. The Angular App receives JSON objects via simple GET requests to the Node.js API.

I realize that it's intriguing to offload major parts of the App to the client, performance wise, but it looks like a pretty substantial leap in complexity as well.

So, does it actually pay off to construct an App in basically two parts instead of one? Or, when is it reasonable to do so, and when is it not? The entire Express/Node/Hogan structure just works so seamlessly together.