You are right - the lack of use cases is contributing to the lack of direction. Maybe what I'm asking for here is "How do you want to use something that generates PDFs from a layout + parameters?" ...
Maybe I do have an idea after all. What would make it the most easy for me (and many others) to use is to allow CSS + HTML to be a feed interface.
I would be completely overjoyed to be able to just use the media="print" within an HTML page to drive PDFs. I realize this is a daunting suggestion that may not even be a good idea but it would be sheer genius if it could be done. I don't know the first thing about PDF internals but perhaps it might map fairly well...? Or maybe there is already an engine out there that could handle the interchange?