Driver Sweeper? Can nVidia/AMD not write a proper uninstall script or what?

What's up with all this driver sweeper fuss? Wherever you look you are told to use driversweeper to remove your old drivers, but why is that necessary. I see the point when for example swapping cards especially if from nvidia to amd or the other way around, but why do that when simply updating? What makes you think that nVidia/AMD are unable to write a proper uninstall script to remove obsolete files during the installation process? I'm just saying my secondary rig is running an 8800 GTX and I've been simply overwriting for four years now and still get THE SAME FPS in unreal tournament three.

Cheers,
Mark

Generally I only tell people to use in it there is a problem with the driver and driver sweeper will make 100% sure it is completely gone.
Its a just incase really.