But the thing is & the points which you all seem to over look & not for reasons of comprehension is that the so called Nvidia specific implementation does work on ATI cards but has been deliberately disabled.
The Nvidia specific implementation is guaranteed to work on Nvidia hardware - that's why it's enabled in the game. The fact that it works on ATI hardware is more than likely a convenient bonus, one that could break at any moment with a catalyst driver update.
If ATI can guarantee that the solution remains functional in future drivers, I'm sure the devs will have more confidence in enabling it on ATI hardware.