I feel that we are ultimately not going to need much of this discussion.
We are just about to announce custom sidebar sections! What if long term all sections are custom sections? … you get to pick titles and which links you have in each bucket.
Want to mix up 3 tags, 2 categories and your bookmarks in one section… go for it..
I do though strongly support @awesomerobot here. I don’t want to introduce any new “entities” into the sidebar. The consistency of collapsible sections is a huge benefit here. Introducing a new entity "non collapsible section with no title, makes me uneasy.