The advantage of using the 'all' property is that big companies like Twitter or Google could simply provide us with a CSS stylesheet that formats a specific HTML to look like a tweet. Then we (as developers) would be able to manage the download of the tweet server-side. This will lead to the fact that the client has to make much less requests to third-party servers -> faster site.
I may have struggled a bit with formulating my idea behind this whole scenario. I will try to do that better in the future... :)