There is great value in fully solving the multipass problem. It will mean that any conceivable pixel shaders we write in the future will work backwards-compatibly on past hardware capable of full multipass temporaries spill, it will just run slower and require more passes on that hardware. That is a HUGE improvement over the current situation, where shaders we write for DirectX9 hardware simply don't work on DirectX8 hardware, and code we write for DirectX8 shaders won't work on DirectX7 hardware.