When you've serviced over 500 machines, then you can say, yes, I know that the plastic bobbin case machines don't have as good stitch quality as the ones with metal bobbin cases. It's not just the bobbin case, it takes into consideration all of the other plastic parts in the machine, and the tolerance level of the factory that the machine was built in. If the needle bar has forward/backward wiggle, then you can't control the needle/hook distance, which is an important timing setting. Stuff like that makes a difference. Probably doesn't make much sense to anyone else -- but I'm a sewing machine mechanic and it's what I look at every day at work.