I think ATI makes a valid point that if the game you like is not a mainstream game that sells a million copies and has benchmark results posted in magazines you will never see this level of optimization from Nivida.
So if all you play is mainstream games your choice is ATI/Nivida, if you play some of those games that are off the beaten path then I guess ATI is the only way to go for overall performance.
Unless someone else has a better explanation.
Not exactly. For one, since we generally don't have benchmarking results for these "less-played games", we do not know exactly how performance differs. Also, it can be argued that general compatibility and driver stability is just as important (if not more important) as raw performance, especially with some less visible older games.