Originally Posted by joe'smom

The 'ditch' is created when you press to the side, leaving one side of the seam higher than the other. You then stitch very close to the higher side. Stitching down the middle of an open seam, you'd be stitching on top of stitches only, not on fabric. You therefore wouldn't be stabilizing that fabric, and could potentially weaken those seams by breaking the original stitches.