One of the main reasons for using a straight piece of hair canvas and pleating it across the back is to give the kilt stiffness parallel to the pleats but allow it flexibility across the pleats. This keeps the kilt from buckling and developing unsightly horizontal rolls in the fell when you're wearing it. You would do this even if the kilt didn't need shaping because the waist and hips were the same measurement. Even in a kilt like that, you'd make the pleats in the canvas.

The lining is structurally less critical, but it's a load easier to stitch both the bottom and top of the lining edges parallel to the grain of the fabric, and it looks better, too.