I don't use a walking foot when I use a serpentine stitch, but I heavily starch backing fabric and also spray baste quilts so that may be why I don't need the walking foot.
When doing wavy lines with a walking foot, I am actually using a straight stitch and simply guiding the quilt side to side to make long, curvy lines. This is a completely different look than the serpentine stitch.
I wonder now if there is some confusion about "wavy" versus "serpentine".