Funnily enough, that is why I like nvidia.
When I first got into computers, gaming and gelahics, they were the new kids on the block in a world of giants like ATI and 3DFX. They were the underdogs, far smaller budget. They succeeded by simply making the best cards AND supporting he industry standard of OpenGL and DX while the other companies were largely offering their own proprietary low-level API such as 3DFX GLIDE and S3 Metal.
Moreover, I used open-source Linux at the time heavily, and still do. I had an ATI Rage Fury, and boy did it make me rage furiously. The Linux drivers were shocking, the openGL support in Linux and windows was dire. Even as an amateur OpenGL programming making a clone of quake3 (everyone did back then), I was finding driver bugs for ATI that some ATI engineers on the OpenGl forums would acknowledge and pass on to the driver team. At first it felt kind if cool that a newbie could be helping ATI make better drivers, but the fixes never really came, the drivers continued to suck and in general it was a nightmare in Linux or windows.
I swapped out for a Nvidia TNT2 and the difference was night and day. Although the Linux drivers were closed-source they worked perfectly, and I really didn't care if a company made their drives open or closed source as long as they worked. Massive performance boosts in OoenGL. Over the next 15 years it has pretty much always been the exact same experience, get a laptop with an ATI/AMD cars and the OpenGL drivers sucked, Linux drivers sucked and the windows DX drivers ranged from poor to good. Every nvidia card had awesome drivers, incredibly fast Linux with solid support.
Then there is the hardware, I've had 3 graphics cards fail all ATI/AMD, the most recent being in my 27"imac.
Because Nvidia is still by far the best choice for gaming and development with the open source API OpenGl on open sources operating systems like Linux. The fact that Nvida Linux drivers (until recently) were closed source made absolutely no difference to me, they worked and let me use the open source OS and open source cross-platform API I wanted to use. that Is why I prefer Nvidia whenever possible. But I'm not a fanboy, my main computer is an imac with a AMD 6970M. The other computer runs kubuntu with a nvidia gts250.