Cut-N-Drill-Parts does not sell metal shielded bearings.  We offer sealed or open bearings. Metal shielded bearings will dent.  When a dent in the shield occurs, the shield will pull away from the bearing race.  That creates a leak path for dirt, dust and debris to get into the bearing and cause a premature failure of the bearing.  That same leak path will allow the grease to leak out of the bearing.

A severe dent in the shield may create a contact point for the ball bearing and cage assembly to rub against the area where the dent is.  This will cause the bearing to not turn smoothly.

A bearing with polymeric seals will not dent.

OEMs will use shielded bearings because they are typically less expensive than sealed bearings.