Puh...I'm not sure if I like that. Yes for skimming a text it might be useful, but then why not just write a very good description/summary at the top or bullet-point conclusion at the bottom.

Also the first sentence of a paragraph should provide a summary of the following paragraph if following good writing guidelines (kinda http://www.une.edu.au/tlc/aso/students/factsheets/paragraph....)

On the other hand, collapsible blog posts could help the writer. For instance after writing a blog post I would love if in iAWriter or WriteMonkey I could collapse all the paragraphs to just their respective first sentence. If the collapsed text would still bring my argument across I did a good job, if not I'd have to rework it.