I strongly agree with the original message regarding the four types of user documentation. Asking that question is what an excellent tech writer does at the start of a contract or job. 
Writing four different types of documentation of course requires more effort than writing one. But in my view the key message is that knowing who your audience is, and the purpose of your document, leads to a much better result for the target audience and simplifies the writer's work.
It helps you consciously think about which kind of documentation will be most valuable, for a given need you want to satisfy. It helps you prioritise work.
Regarding Dave Barton's perfectly valid concern, about resourcing all four types, resourcing is always a problem. However one of the key strengths of communal efforts to create something, especially from volunteer work, is that work can be shared out; schedules are often flexible; and users often appreciate the volunteer efforts.
Managing communal projects effectively is a bit of an art. Jonathon's post about the barriers to contributing made me feel that here was a kindred spirit. I personally certainly felt I needed a combination of maze exploration, stubbornness, and persistent digging to find my way here.
Some discussions (and some trickles of effort), toward more easily funneling helpful contributions towards the documentation could pay off enormously. I think that's a separate topic, however.
My 2c.
luke
PS:
When I wanted to contribute to this thread, I hoped that the "Reply via email to {Pixie}" link at the bottom of the archive page might do what I wanted:
https://www.mail-archive.com/documentation@global.libreoffice.org/msg12948.html
Instead, it opened up a no-subject line addressed to pixie dot dust at bk dot ru (IYKWIM).
Here's hoping this message is successfully threaded just by the subject line!