Why they promote these weird technologies instead of just supporting OpenGL quad buffering?
Well they say AMD cards beginning with HD6000 support OpenGL quad buffering, yet HD3D is still what you see on the front pages (well, maybe because there is no native DirectX quad buffering support yet)...
Two reasons: Keeping an incentive for professional users who need quadbuffer stereo to buy the professional cards. Now with 3D Vision being pushed so hard a lot of people asked "uncomfortable" questions. The other reason was to try attempting on Vendor Lock in with a custom API, so that 3D Vision games would work only on NVidia hardware.
Similar reasoning on the side of AMD. However FireGL cards didn't keep up with the Radeons and so there's little reason for AMD to make their Radeon cards less attractive to professionals (current AMD FireGL cards can not compete with NVidia Quadros, the Radeons are also the competition for the Quadros), so having quadbuffer OpenGL support for them was the logical decision.
Note that this is a pure marketing decision. There never have been technical reasons of any kind for this artificial limitation of consumer cards.