I use TBX an exploratory intermediary between the data bucket of DT and ... wherever my notes are going. I use the movement from one app to the other to refine and rethink.
I use DT as an inbox because I typically take notes on an iPad and phone as I read. I save them to a general inbox and later sit down ad sort them into areas where I expect to use them. I teach, so it's classes mainly, but I'll create ad hoc files or categories within files for what I'm reading at the moment. Oh, for an iPad version of TBX ....
For a small writing project like a blog post, I'll start with the raw note in DT, export it to Ulysses for markdown formatting, and publish to a blog, making refinements on the way from note to Ulysses to blog post. With each move, I re-consider where the note is going, what I'm doing with it, and tailor content. Once the post is published, I'll bookmark it I'm DT with a link to the original note. As Mark B points out, storage is cheap. Links are even cheaper - although cognitively expensive. The original note remains in DT, but I now have a variant.
For a major writing project - a course (I consider teaching a face to face course a writing project), the draft of a paper or presentation - I'll move notes from DT into a course-specific TBX file. Drag and drop. As others on this forum seem to do, I revise notes that I move into TBX to check my understanding of what I've read. I use TBX map view to organize and conceptualize the course and weekly sessions. Some maps become class presentations, although I rarely find the time to fashion these into stand-alone work. They stand as documents apart from what I have collected in DT, even it they are unfinished.
I use DT to collect and TBX to gather and organize. DT is a commonplace book - an Ur-Notebook. TBX becomes a way to circulate Work in Progress beyond the blog post.
In creating these TBX maps, I'll typically draw from a wide set of DT files and categories - dragging stuff from afar that seems to fit to see how it might. DT search, concordance, and see also help locate these notes. I'll also tap into DevonAgent at this point to look for connections Out There.