Doesn't Windows Vista and newer already come with a D3D9 based OpenGL 1.5 implementation and WARP software render device?

no, they dropped that plan

vista and win7 have the same opengl fallback that windows xp had. both xp, vista and win7 have a software rendered opengl fallback, and a special d3d9 based fallback which is deactivated. this last does not work with generic softwares at all, becouse its deactivated and not oficially exist, it cant be accessed, only can be turned on by installing some special tools to allow it. the opengl software renderer of microsoft, which is the generic fallback, can render a few thousand polygon around half or 1 fps even on the newest core i7 cpus, so it not really works in practice, and does not works with generic games or softwares at all. microsoft wrappers supports opengl 1.1 without any extension except to control vsync and abgr, and they crash from the most texture formats or even from the most basic functions. microsoft have this fallback unchanged since ~2001. in practice, both its useless, no software is compatible with them. microsoft was unable to deliver they opengl 1.5 wrapper/implementation at all, they discontinued the development after a few month of struggling. they opengl 1.5 plans are not available any more. http://msdn.microsoft.com/en-us/library/windows/desktop/ee417756%28v=vs.85%29.aspx

(the case is the same with directx, they originally planned to have a fast and good multithreaded software renderer fallback for directx, but they cancelled that too, software renderer only available after installing directx sdk only after special linking and initialization of the software, its cant be used in practice. also thats 0.5-1 fps so its useless in practice again)