It's user experience. Without a simple way to control the "page" it makes very hard to navigate to a point you want and you know it exists before hand, for one. Ideally that navigation box would be always visible somewhere. And it would probably be the most difficult thing to build in this solution.
So, to get something done, I can sure appreciate less work needed in not doing it, but it will not be a good UX at very least for some expected usage cases.
Also, the browser scrollbar won't be able to do this job, unfortunately. Because the full page won't be loaded at first, or sometimes at all.