Rubberized undercoating is a sacrificial layer, put there to help stones bounce off without chipping the paint finish. Normal wear and tear destroys undercoat over time, in areas where road debris impacts the body. Proper maintainence includes re-application in worn areas periodically. If you have undercoat on your car it should be touched up every year or two, in the fall season. Handy aerosol cans are available for this.
The cars have an inherent weakness in the factory-applied corrosion protection between the walls of the inner and outer rocker panel structure, so many times rust starts on the inside of the sheetmetal where it can't be seen until too late. As ITHurtz mentioned, they also have corrosion hotspots where road debris can collect... several places down low on the car. It is very important that these areas be cleared so moisture cannot accumulate.
