When I was training to be a teacher, I worked at a before and after school program that had consoles. The cd's and dvd's were extremely scratched. They can stop playing... even the black bottomed ps1 discs.
From what I understand of how the laser reads, it is mainly the scratches you can see when looking dead on straight at the disc (parallel to your face, basically). This shows far fewer scratches than when rotating the disc at angles when inspecting. This is probably why a disc can still be read well when there appears to be several scratches. Some disks look bad at an angle but pretty good straight on.