I don't agree with your comparison with PDF renderers. The key difference here is most PDF software is just what you mentioned - rendering. Adobe don't need to worry (much) about creating PDFs, because most people use Word processors to do that. All they need to worry about is a bit of translation (sometimes from another spec they own - PostScript), and the rendering side. This makes for a far easier world to keep everyone ticking along with new functionality. To me, PDFs are a secondary document spec - great for rendering, but of no value to the creation and development of documents, and this is reflected by the number of word processors you see out there that actively store documents natively as PDF.
I also believe, in contrast to your assertion, that Office document specs are transitory. They have an (albeit, slower) evolution process due to the evolution of the Office productivity tools used to maintain them.
As an example, if you take a look at the user interface changes implemented in Office 2007, regardless of what you think about the ribbon, you'll notice a proportionally higher emphasis (quantified by screen real estate) on tools that define documents.
Prior to Office 2007, and in fact in most competing, current, 'mainstream' versions of word processing tools (e.g. Open Office), UIs pushed people towards creating non semantic documents. You want more emphasis on a heading? Up the font-size, and bold the font-face. With MS Office 2007, the UI not only gives you a whole bunch of styles, but shows you what they look like so you are more likely to choose them to obtain the effect that you want. This - to me - was a real coup for Office.
The document format they use has evolved with the product. Do a bit of research, here on slashdot even, and you'll find out / be reminded that the .DOC format is legacy from the old Word 6 days, where changes to documents were actually appended to the bottom of the file in some case, so that time to render documents ready for editing was kept efficient (this was back when 486's running Windows 3.1 was the norm). Microsoft made a huge leap that began in Office 2003 and culminated to something really helpful in Office 2007.
I'm from a web development background, and I understand the need to create semantic-rich documents regardless of the medium in which they're published. With 2007, the docs suddenly become far easier to maintain in the long-run.
I saw the change Microsoft made with this particular component of Office's UI as being a genuinely good thing for the future of document management. And its my belief that until other office products come up to speed with this, they'll continue to remain where they are in terms of marketshare, or possibly go backwards.
What Microsoft did with Word 2007 is update the document standard profoundly. Now you can argue against their non-Free attitude all you like. But, purely from a non-political, technical standing, the OOXML spec is much more Open than their previous document formats (e.g. .DOC), and they are much better than where they were.
With the example of a system that may take the contents of a document, and re-purpose that content for the web, I'd be much more comfortable with Word 2007 than any other word processing products, for the two aforementioned reasons: their UI pushes users to create well defined documents, and their underlying format is far more readable, and closer to HTML, than previous Microsoft formats.
This is how Office productivity software has changed. The time scale of this change is far greater than web standards, and that is because the user base is far less open to change. It's still quite common for users to be on Office 2003, and they blame the UI for not upgrading, which, from a wholistic point of view, is exactly why I think they should upgrade.
I think you're right - Office documents "shouldn't be" transient and fluid. Perhaps I'm being optimistic, but it's my belief that they won't be needing as fundamental change in the future, now that we have the OOXML format. The only fundamental change I'd be happy about perhaps would be to bring ODF into Office 2007 natively, however I don't think Microsoft are going to back-down on their positioning with the OOXML format now.
I don't astroturf for Microsoft - in actual fact I produce and present a tech oriented radio show on a local community radio station, and I'm often critical of Microsoft on said show. One of my co-hosts is a developer evangelist for Microsoft, and while I'm diplomatic when I have him on, the conversation can be considered sometimes "terse" to say the least. And yet he keeps coming back :)
In technology, particularly when it comes to Microsoft, I tend to be a pragmatist. I find Microsoft's previous tactics in numerous circumstances to be abhorrent - particularly the browser wars. I also think they make some good software - namely Office 2007. I don't think they make good OS's, although Win7 was an big improvement.