FTPOnline.com: Udell: Longhorn & Yukon's XML-RDBMS-CLR Triad "...this excerpt from an interview with Quentin Clark, director of program management for WinFS:
On the object/relational/XML "trinity"
Why do you need all three? I take it that it's obvious why you need objects: you program to them. The CLR has given us some language independence, and we've done a fairly good job building an object universe better than we had before, we're strong believers in that. As for XML, there's no argument there either. The big thing isn't turning out to be industry schemas, but the fact that you have this self-describing thing, this is what I can learn about it, and I can reason on it in a programmatic way by pulling it up into an object.
Then there's relational, this is harder to describe. I will observe that nobody has built an XML store that has the level of scale, performance, or capabilities of today's relational stores. It's just true that the relational model has a set of design characteristics that give it performance characteristics that are just inherent. Doing things in the XML store doesn't give you the same benefit—and that's not even accounting for the fact that there's so much data in relational stores today."