> 1) It helps you keep your place -- you can bookmark one of the 15 pages to continue reading from later on, or forward to your friend ("check out the third paragraph" instead of "84th paragraph"). Obviously, you'd never show a 100,000-word article on one page (except for printing). At some point, pagination is necessary. Where to draw the line is obviously depends on a multitude of factors.
Put permalinks next to sections headings and/or change the fragment in the url bar with javascript as the user scrolls. Links work, and no need for pagination
>2) Feeling like you're "in" the site, not in no-man's land. From a web design perspective, you always want people to be able to navigate easily to other parts of the site -- you want stuff in the sidebar, never be too far away from header/footer, etc. You don't want people to be in a narrow column of text, with total white on both sides and nothing else. (Obviously users might often prefer this, like I do, hence Instapaper etc., but other users also like to be able to skip around to the rest of the site easily once they decide this particular article isn't for them, etc.) Of course, "floating" sidebars, headers, etc. can help alleviate this, but this requires the sidebars to be very small, which is not always possible.
Use position: fixed (as you mention).Having submenus that expand on click alleviates the space constraints. You can also swap out content in your header or sidebar as the user scrolls (but please use tranitions, or you'll give everyone eyestrain).
> 3) "Chunking" large pieces of content is just a good thing. A huge wall of text is just no good from a usability or legibility standpoint. This, of course, is why articles are broken down into sentences, paragraphs, sections, parts, etc., so people "know where they are". Now I'll be the first to admit that paging is a particularly inelegant sledgehammer way of chunking, but when you're dealing with lots of articles of different lengths, formats, etc., it's a kind of lowest-common-denominator that is seen as better than nothing
What's wrong with section headings?