Pleats are interesting.
I have noticed them in tailored vintage patterns as a way to add movement ease while still keeping the slim profile.
A few years ago I came across a pattern for a narrow skirt with a front box pleat and was amazed to see that the pleat, which I would normally have expected to have separate pattern pieces, was made by just laying the centre front of the skirt front 3" away from fold, stitching a centre front seam down to the pleat opening, pressing a box pleat, topstitching the seams open above the pleat, and hemming.
You can see the end result here to the right in a skirt that I have worn and loved for at least two years.
I use the same method to make knife pleats at the bottom of skirts, although I don't leave the wide 3" pleat fabric up the length of the skirt, like I did in this vintage knock-off. For skirts I just lay the vent extension on the fold and pivot out a stitching line to the fold, like I have here:
The key to keeping the pleats sharp is to top stitch the pleat fold(s) close to the edge on the inside right through the hem.
Let me know if these instructions are clear enough. By using the fold, and eliminating the seams of pleat pieces you get a much lighter, more elegant finish. And because the fabric is on the fold the grainline will be perfect, which is essential to the hang of a great pleat.