Indeed, but then the URL doesn't change, so the page can't be emailed, tweeted, etc. The hash-bang allows you to change the url, use AJAX to change the content,

Indeed, but then the URL doesn't change, so the page can't be emailed, tweeted, etc. The hash-bang allows you to change the url, use AJAX to change the content, and retain browser history. The history API in HTML5 allows you to accomplish the same thing without the hash-bang, but browser support is limited. I favour using the history API for progressive enhancement and defaulting to traditional page requests.