3DGameStudio has historically used the DX version that was in use by the majority of the game playing public. When (and if) the time comes when DX10 is in use by the majority of gamers, 3DGameStudio will most likely add support (it would be crazy not to).
(Note: The same holds true with OpenGL, LinuxOS, Mac, etc. There isn't anything random about it, 3DGameStudio goes where the users are.

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Currently, the number of users that can run DX10 is somewhere between 1 and 8% of the total Windows market. Way too small for now.
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2. new possiblities. play crysis. first render it in dx9. then render it in dx10(using the "-dx9" in the target box) youll see the possibilities.
I think this web-page sums things up nicely.
There are advantages for using DX10, but most of them are on the programming side (the stuff 3DGameStudio handles).