I had a Kenmore from the 90's that DIDN'T come with a straight stitch plate but DID have L,C,& R needle positions. The manual said to use the left needle position for straight stitching on light weight knits, silky fabrics....any fabric that might have "flagging" issues. Since you were only using the far left side of the zigzag plate, there was more support under your fabric. So maybe there was a reason other than cost. I've always heard they were cheaper to manufacture, too......