Take a simple 5-page site that just covers who you are, what you do, some example projects and contact info or something. I bookmark one of those pages so I can read it later when I have more time. I get home, open the bookmark and the page loads with only the relevant HTML, CSS and JS. Done.
Say that site is converted into a single-page, super-scroll site and the content equivalent to the page I bookmarked happens to have been placed at the bottom of this page. Now, when I open the bookmark (assuming there is a 301 set up correctly) I have to wait for the content from all the other pages to load as well before the page is ready. While this is happening I might start reading my content only to have the page grow as images load in, pushing my content down and making me lose my place. There might be a larger amount of CSS and JS as well because previously page-specific code will now need to be loaded every time, regardless of if it is needed for the content the user wants, which could hurt functionality.
It i