That's simple; there is no common format today. If the OS manufacturer doesn't set an example why should anyone else.
However if there had been a WinFS common object format, with scope for extensibility then you may well have seen it used.
Blowdart, I think that you're missing Brandons and my point. There are three options to how to handle the schema:
1) MSFT defines a fixed schema (or a base schema). 3rd party vendors scream because the base schema doesn't meet their needs.
2) The first app installed defines the schema. There is no interoperability because there is no standard for the schema. [12,3] doesn't really work very well as an email address these days.
3) Each app extends the schema in their own way. There is no interoperability because the applications don't share information. Or worse, app A reads the info from app B and modifies it. But app A doesn't understand that app B has actually tied two separate
fields (the base field and an app B specific extension) and app A has just corrupted app B's view of the schema.
Marking apps as "WinFS aware" doesn't help, app A and app B are both WinFS aware.
Building an open extensible shared common object model is a nightmarishly hard problem, that's why (to my knowledge) nobody has built such a beast before. The best that's been done is to define fixed export formats and people interchange data using those.
On the other hand, when there's a low fidelity mapping, you get things like
I'm not saying that the problem can't be solved. I suspect it could be. But I'm saying that this problem is crazy hard.