The Next Stage in Composition -- McIlroy
Nearly all of the XSL-FO renders offer print output via PDF. It seems odd at first—why PDF? But what alternative? QuarkXPress native format? OEB (Open e-Book)? PostScript? No, PDF is the logical format. It's well-structured (much more so than PostScript), and well-documented (the 1,172 page PDF Reference for PDF 1.5 can be downloaded without charge from Adobe's Website).
Though controlled by Adobe, no one is prevented from using it (nor required to pay a royalty for doing so.) It's the ultimate page-oriented print format, and a completely natural output file format for XSL-FO documents.
OK for Third Parties
Adobe has embraced this FO-PDF workflow, and broadly endorses it for third parties. I don't know whether to read this as a win for PDF or as Custer's Last Stand. In my view Adobe continues to struggle to find a clear role for PDF in an XML world.
Encompassing XML within PDF seems natural until you question the bottom-line benefits. Is the XML document provider more fortunate to have PDF to represent document appearance, or is the PDF user more fortunate to have the granular markup provided through XML? There has been a multi-year movement within both the XML and PDF communities to support the proposition of PDF and XML, rather than PDF or XML.
I remain unconvinced.
However I think that the question of the importance of XSL is perhaps more so related to the question of the ultimate importance of XML.
The key value of XSL is that it's contained within the family of XML specifications, and adheres to the XML syntax. As such, it is potentially able to offer two advantages that were never available to SGML. The first is the innate ability to tie the appearance aspects of the publishing process with the workflow and commercial aspects of the processes, in a single data stream. Standards like JDF, AdML and NewsML arose during the XML era, not the SGML era, and promise enormous workflow and business benefits.