That was removed from gnome-screensaver many moons ago because the GNOME folks decided that options were too confusing for users to handle. I immensely disagree with them, but they seem too hardheaded to change their stance on it any time soon. Still voting up.
It should really really come back. Gnome should never prevent the user from customizing. It should only make very good defaults, and not take the control away from the user!