Yeah, solar thermal is woefully underused. Even something like attached south-facing greenhouses and solar hot air panels can cheaply retrofit passive solar design onto the existing building stock.

To that list of rocket stove applications & advantages would add backup heat for solar hot water, accessible to developing countries and the poor, and (when built from masonry) ease of construction in the charred afterscape. I'm kidding of course on that last one. Mostly. ;)