If your personal preference is to spend more money on something for less performance just because you like the company and don't want to switch I would call that fanboyisim, lol.
Performance isnt always the issue.
Theres many other factors involved, such as drivers, looks, and warranty.
I am by no means a fanboy, but i prefer Nvidia and XFX cards because of of the lifetime warranty, black pcb (which fits the color scheme of everything else in my system) better drivers, and folding performance. To me, these reasons are valid enough to sacrifice some gaming performance (which is very minimal, and when it comes down to it the difference doesnt matter much).
Same thing when someone buys a luxury car over a sports car. The sports car is faster, but personal preference leads the person to buy the luxury car, simply because thats what they like.
I know i pay a premium for wanting an XFX and Nvidia card, but im fully aware of this. If i were looking for the best price/performance, then of course an ATI card would be the best choice.
And ive owned both Nvidia and ATI cards( in fact i just sold an x1650gt about a month ago), and i have an AMD system and i just ordered an E1200 and Intel mobo, so by no means am i a fanboy.