I don't need an Nvidia driver for the computer to work if my video card is an ATI.

Suppose you change video card ;)

If there are drivers for a hundred different makes and models of viseo card, and you only have one card in the system, that seems to be a huge inneficiency, taking up disk space and possibly memory, and a possibly a hundred "if" statements to go through for the driver actually needed to initialize.

That's where modules take place. Only the driver modules you require are loaded. You can even delete from your harddrive the modules you don't use. It won't take any ram space the modules you don't use.

The fact that you can make an installer that only installs on your computer the modules you require, doesn't mean that there is no source code for the modules you don't use :p