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.