I spend a lot of money on books. O'Reilly, Packt, etc. put out some very useful books documenting FOSS tools (and for statistical computing in R, both Springer and CRC publish a lot of really excellent books, although they can be very pricey). These books are mostly written by people who can really write and edited by people who can really edit.
And when I spend my money buying this kind of documentation, it supports a culture of good technical writing and ensures that good technical writers to get paid for their work.
It might be nice if there were a similar quality of documentation available for download under CC, but absent that it's nice that there is a thriving industry providing good, useful documentation in books.