The only really appealing reason would be DX10 games, but there's maybe three games that take advantage of it. Plus, it obliterates any backwards compatibillity it used to have. :-(
You're listening to too much noise.Most games that run in XP, will run in Vista. This odd rumour of "everything before DirectX10 will stop working" refers to the DirectX SDK which no longer supports older versions of DirectX. Yet the entire point has been distorted somewhat in a Chinese whisper-like fashion.