It's called personal preference, but then there's a huge amount of people that will only buy NV because they don't know any better. The biggest thing NV has going for it is brand awareness.
Some people who don't know any better, would buy an 8600 and be chuffed with them selves because they've got an NV 8 series. Not because of its performance.
Most people who don't have a clue will think NV cards are the only graphics cards, and I've experienced this first hand many many times.
A friend of mine is looking for a 'budget' graphics card for around £120 or so, I've recommended him a 4850, but he's got some n00b friends who are telling him to get lowly NV cards because they're 'better'.
Well, he's very interested in the 4850 anyway, and is hoping to get one soon.