Today's most formal swag treatments are typically rather fixed and perfect, in neo-classical proportions. This image is an example, although French inspiration is also very clear.
Historically, trim reinforced seams and covered hand stitching in a way that appeared more perfect than the seam itself. We don't need to reinforce or perfect today, but the other reasons to use trim remain unchanged: visually separating fabrics (especially the front and back of jabot folds), emphasizing the lyrical edge of the treatment, and contributing to the ornamentation or personality of the design.
Here the arched top of the treatments, while impressive, aren't bold. Softly colored blue and green fabrics remind us of painted ceilings of romantic rooms. No edging or ornamentation define the arch shape. In terms of today's window treatment design, the arch top is an upscale element that is intended to be quiet and refined. Some might consider the cost of the curved top to be greater than the impact they provide. This may be exactly the rich, understated look desired.
