Moving interactions to JSON in many cases is just a better experience. If you click a Like button on Facebook, which is the better outcome: To see a little animation where the button updates, or for the page to reload with a flash of white, throw away the comment you were part-way through writing, and then scroll you back to the top of the page?
There's a reason XMLHttpRequest took the world by storm. More than that, jQuery is still used on more than 80% of websites due in large part to its legacy of making this process easier and cross-browser.