The individual sites using it would load faster, since much of the code would be cached, and be easier to use, since most users would have encountered the navigation before. And novices or those with little time for development wouldn't have to re-invent the wheel (or mouseover, as it were) each time they needed to deploy a solution. I realize that this already happens in a defacto sense since so many people steal or borrow scripts already, but why not formalize it? Perhaps package all these scripts like Java does with applets and have it ship with the browser?
Of course, it'll probably never happen, but it would be very net-like, since it would be inherently open-source and extensible. Anyone wanna help me make such a site? Or better yet, make it for me? Let me know.