The problem jsHub is trying to solve is not "only ever load one piece of JavaScript in a page" it's "only have one piece of JavaScript for tracking, ad-serving, behavioural targetting, ...".
The reasons to do that are laid out in my blog post but the most important is consistency. If one piece of JavaScript gathers the data once and passes it to various vendors, then you know that the same data was sent, and was actually sent.
The reasons to do that are laid out in my blog post but the most important is consistency. If one piece of JavaScript gathers the data once and passes it to various vendors, then you know that the same data was sent, and was actually sent.