Is there any particular consideration against using the root node of the HTML document as a message bus for a web application? If this question is too generic please consider the case of a polymer MVC application.
As far as I can see the document is always guaranteed to be there and is easily accessible from everywhere, even from within the shadow dom. By contrast Polymer docs seem to want people to use a dedicated bus element that can be injected where necessary. In other words I'm not quite sure what advantages this approach would provide.