I'll weigh in here: We may be very atypical Unbounce user, but we use it less for "landing pages" in the traditional sense, and more as an extension/addition to our website. Unbounce allows us to quickly create microsites or pages for specific events or topics that we'd otherwise have to get our developers to code and release to put on our corporate website. Using Unbounce is so much faster, easier, doesn't require as much technical prowess, and we can make sites/pages that look pretty much like our website.
Collapsible sections would be great functionality, especially when making pages that aren't really intended to be "landing pages" with a specific call to action, but more informational. For example, we'll often make a "microsite" for a conference, which might include a few linked Unbounce pages, and being able to collapse sections like "Expand to view conference sessions" or "Expand to view exhibit hall floorplan" means that the site itself can look nice and clean/streamlined, but we can provide more detailed information if people want it.
Hope this makes sense!