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Building an infrastructure that uses and supports all of the Direct X9 specification allows for a lot more than just "wobbly windows" as hardware progresses.
As hardware progresses? No effect that's usable on a desktop has so far been dreamed of that uses even a fraction of the power of today's midrange video cards.
Designing a system now, in anticipation of graphical effects which would have to be so ridiculously advanced compared to the current class of ideas, and locking out millions of people in the process, is a mistake.
Do you anticipate Longhorn having any effects requiring vertex shaders? So far the demo's we've seen are nothing more complex than the usual "windows blowing in the wind" type effect.
To the end user, none of this matters anyhow. They will see the effects OS X does now, on cheap video cards, and they will see the effects that Linux does (assuming this goes anywhere) with the same class of hardware, and then they will see that Longhorn does the same effects, but requires a DX 9 level card.