Huh? Howzzat work? I agree with the brightness part, but how can a focusing screen change the apparent dof on the screen?
As for the exact optical reasons, I do not know. But I know it has something to do with the design of the little fresnel (or whatever it's called) pattern cut/ground into the screens. They are designed to focus and optimize the viewfinder image for f/2.8 or slower lenses, because they figure most people using AF cameras are using zoom lenses.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."