The broken plastic tab keeps the pickup roller in position so it can't short out against neighboring metal parts of the truck. With it missing, the pickup roller has a larger range of movement and can then short out creating the pitted weld marks on the pickup roller. This shorted current can also weld the rivet in similar fashion.
Most often, I see this type of failure attributed to some sort of mishandling or poor storage, but sometimes a particular gnarly derailment can tweak or trap the roller really badly, say in a switch frog or between rails.
Bottom line, it isn't the end of the world and a fairly easy fix.
