I agree, and I think I was suggesting the wrong use case at first.

In a typical blog post, the summaries shouldn't be necessary. A good use case for Collapsible Blogging is when you are listing multiple items or events, and the reader may only be interested in reading about one or two. Then the reader can choose to expand just those. May be better as Expandable Blogging rather than Collapsible Blogging.

I'm going to try it out this way and see what people think.

That's very true. I had a very similar idea a while ago, but it worked the other way around: semi-relevant tangents and sidenotes would be collapsed by default, and you click to expand them.

A bit like an inline footnote?

Very much like one, just intended to be used more liberally with other related content as well.