Your graphics card doesn't "need" DVI-I. Dual link DVI-I can be fanned out to dual and single link DVI-I, dual and single link DVI-D, and DVI-A. Most graphics cards feature a dual link DVI-I because it's a universal connector that can be converted into a classic DE15-F VGA connector by tapping the analog parts, or HDMI/DisplayPort by tapping the digital ones.
More info:
http://en.wikipedia.org/wiki/Digital_Visual_Interface#Connector