When you have a persistent page (that you update with HTML5 push-state) you suddenly have to worry about issues like memory fragmentation. If you do lots of small updates to build a page from a template + JSON every time the user navigates it's easy to hit some situation where everything starts lagging for no clear reason.
If you simply swap out a large part of the page with some HTML right out of an Ajax request you don't have to worry about any of that.
Let's be fair - poorly designed code causes memory bloat whether you're on the client or on the server-side.
Also, garbage collection efficiency in browsers is a metric of fierce competition among browsers. It's superb already, and it's getting awesomer by the minute. It's a safe choice for the future.
If you simply swap out a large part of the page with some HTML right out of an Ajax request you don't have to worry about any of that.