The metal bobbins can contribute to wear at the bottom of the plastic cases .... or so I've read.
The plastic bobbin cases in the Singers we have all show roughness at the bottom from wear. They vary a bit, but none of them are worn out. It is conceivable that if metal bobbins with pealing plating, rust, or other wear were used exclusively they could wear out the bobbin case. But as I said, none of ours, we have three now, had five, show excessive wear.
Joe