In a recent post entitled 15 Science Street Tim Bray, one of the inventors of XML, writes

Microsoft’s main talking point (I’m guessing here from the public documents) was that their software and format had the advantage that in WordML you can edit documents from arbitrary schemas.

Our pushback on that was that editing arbitrary-schema documents is damn hard and damn expensive and has never been anything more than a niche business.

which seems not to jibe with my experiences. Many businesses have XML formats specific to their target industry (LegalXML, HR-XML, FpML, etc) and many businesses use office productivity suites to create and edit documents. It seems very logical to expect that people would like to use their existing spreadsheet and word processing applications to edit their business documents instead of using XMl editors or specialized tools. More interestingly Tim Bray contradicts his position that editing user-defined schemas is a niche scenario when he writes

As we were winding up, a couple of really smart people (don’t know who they were) put up their hands and asked real good questions. The best was essentially “What would you like to see happen?” After some back and forth, I ended up with “You should have the right to own your own information. It’s your intellectual capital and you worked hard to produce it for your citizens. Sun doesn’t own it, Microsoft doesn’t own it, you own it, and that means it should be living in a nice, long-lived, non-proprietary data format that isn’t anyone’s competitive weapon.”

He took the words right out of my mouth. This is exactly what Microsoft has done with Office 2003 by allowing users to edit documents in XML formats of their choosing. In the letter Bringing the XML Vision to the Desktop with Office 2003 written by Jean Paoli of Microsoft (also a co-inventor of XML) he writes

an even greater and more innovative benefit is the fact that companies can now create their own XML schemas specific to their business, define the structure and type of data that each data element in a document contains and exchange information with customers and business partners more easily. This capability opens up a whole new realm of possibilities, not only for end users, but also for the business itself because now organizations can capture and reuse critical information that in the past has been lost or gone unused. 

Office 2003 is a great step forward in enabling businesses and end users harness the power of XML in typical document interchange scenarios. Arguments about whether you should use Sun's XML format or Microsoft's XML format aren't the point. The point is which tools allow you to use your XML format with the most ease.