Car manufacturers haven't used a points breaker system in a distributor in a long time. As Gasoline fumes said, the rotor doesn't actually make contact with the posts in the cap, it arcs across, but due to the heat generated the posts erode after a while, and carbon arcing begins to occur, which is when you need to start looking into replacing both the cap and the rotor. The cap's primary function is to keep the posts in the same relative location to the the distributor body, nothing more. As for keeping the inside dry, and free of dirt, that is a secondary function manufacturers started using to their advantage when the began to realize how much havok dirt can play with spark. But again, Cars haven't used a point breaker system in the distributor since the 70's.