I think this is pretty cool but you're sharing a Web page rather than "screen sharing", right? That is, it's good for webapps but not for Excel, etc? It'd be cool if HTML5 got an API to do proper screen sharing one day though..
1. It's more private. The rep doesn't see your whole screen, just the contents of the page. 2. It's more secure. We can redact DOM elements with sensitive information like passwords so that they never hit our servers and are never seen by the rep.