I had setup Ubuntu 12.04 on a laptop which was connected to a docking station with dual monitor setup. After installing the nVidia drivers and setting up the screens I found that the mouse would lag when crossing between the two screens. It would require more mouse movement to force it to the other screen. This was very annoying!
The purpose of sticky edges is to allow applications to dock to the edge where the two monitors met. By holding the mouse at the edge it gives the user enough time to dock or to keep forcing the mouse beyond the point of resistence to get to the other screen.
Interesting concept and nice way to working around docking to the edge where the two monitors meet but I am not a fan.