I believe I understand the concept, and the purpose, but this seems limited to specific types of web apps. General sites (news, blogs, and most services) - this approach would not make the most sense, since JavaScript interacting with the DOM does not compare speed-wise at this time.
This would work well for example and demo sites (portfolios, resumes, showcaes, etc) and possibly web games. It could be useful for building in-app content, such as if you have a large project like lucidchart.com - you could use something like this to build dialogs, modals, help content, and those types of things easily, in page, by loading a javascript asset on demand rather than running a request for more HTML/CSS/etc.
This would work well for example and demo sites (portfolios, resumes, showcaes, etc) and possibly web games. It could be useful for building in-app content, such as if you have a large project like lucidchart.com - you could use something like this to build dialogs, modals, help content, and those types of things easily, in page, by loading a javascript asset on demand rather than running a request for more HTML/CSS/etc.