Well, the key here is that any non-JSON data you pull from a URL should be static. That means it can be cached, and the client should only be pulling a small amount of data that varies based on their account.

I wouldn't write an interactive web application this way, but for sites that are mostly content the approach works fine. You still have to test on multiple browsers, and you still have to write code that handles the differences in browsers/engines/etc. But your servers are doing less work, and the content reaches the customer faster. It's still up to you to optimize your JS (though I guarantee most tracking cookies are taxing JS far more than rendering a few divs will).