It has to do with the game's engine, not the whole functionality in the hardware.
No it doesn't. Replacing the X1900XT with a 7800GT solves the lack of alpha texture anti-aliasing - and it isn't because the games' engine doesn't allow it.
My Adaptive A-A works in Half-Life 2 and any Source-based games...
Performance AAA doesn't (multi-sampling adaptive AA).
But, as long as your frames are good the better looking Quality AAA works fine. 50% functionality just worked out in favor for image quality for that title.
...Medieval II: Total War, Dawn of War: Dark Crusade, Company of Heroes, The Elder Scrolls IV: Oblivion, Battle for Middle-Earth II: ROTWK and Age of Empires III (all the games I play currently). Don't blame ATi because developers can't use their features properly.
I never said that it is broken in all games, just in a few (and a few I happen to play). I'm sure if one of the games on your playlist didn't work at all you'd feel the same.
Not a decisive con for the X1900XT to have, but still a con. I probably would have gone for the X1900XT as well, especially with that price and aftermarket cooler already on it. However, the 7900GTX is still a good card. It's main drawback is its price, IMO. It really should be a little cheaper, not more expensive.