Brion-
Have you tried the feature? When you click an individual section in collapsed mode, *only that section* is loaded. Thus, no retransferring (except for the page layout).
Then I don't understand the point at all. Use separate pages!
The feature was inspired by the current situation on [[Wikipedia:Votes for deletion]] (en:). The page became so long that modem users complained that they could no longer effectively work with it. (Even though parts had already been split away to other pages.) So some people got clever and started using templates for each section. Now if you want to nominate an article for deletion, you have to create a template page and add an edit link to it. After the debate is over all these templates keep floating around and cannot be easily refactored because they are spread across hundreds of pages. While the problem of slow editing has been solved, the problem of slow loading still exists, and people continue to complain about it.
What are the alternatives? It has been suggested to link to the deletion debates on Talk:-subpages, but the problem with that approach is that it's no longer possible for those who *want to* to quickly view all ongoing debates on a single page.
Similar problems exist on other very long pages. The dialup users complain that they can't work with the pages properly, and the high bandwidth users complain that splitting everything up makes it hard to find and refactor.
By having a threshold defined in the user preferences, we can satisfy both parties without sacrificing usability as the template approach does. Now of course I agree that in many if not most cases, long pages should simply be split up, but as the examples of VfD (and the Village pump, and FAC, and - occasionally - RFA, and ...) show, this is not always an option. And where it is, it often takes weeks or months until someone goes to the effort.
I therefore predict that this feature will be much loved by all. The only possible negative side effect I can see is that people might be discouraged from refactoring long pages which will lead to larger revisions, but of course we should solve this problem by reorganizing revision storage, which is not urgent but should be done for 1.4 or 1.5 in any case, together with the schema redesign.
Regards,
Erik