Game developers are not 'normal users'.
If I'm working as part of a team, we'll have internal systems for collaboration, usually including a wiki.
If I'm working alone... then maybe I'd find a use for a standalone tool like this, although most likely I'd just use OpenOffice Writer. But if the tool was comprehensive enough in terms of search, cross-referencing, indexing, etc., it could be useful. I still wouldn't want an explicit tree structure though.
One thing to bear in mind is that game development documents are communication tools. Is this tool for communicating with the writer, ie. as a memory aid? Or is it to communicate with others, ie. to provide documents to work from? If the latter, what benefits does it offer, given that it will just export to a file similar to that created in a mainstream app?
What would you want rather than a tree structure then to keep it organized and/or categorised?
It's a bit of both. Nothing stops more members on the team using the tool to view the lore structure as intended by the program. Exporting to PDF or similar is for convenience and if that's how the writer uses the program, it would simply serve as Memory Aid.