In practice it doesn't hold up, cake flour thickened sauces break down and become soupy if held at serving temps.Cake flour like Swans Down is like White Lily here in the deep south: both are higher in starch and lower in protein unlike typical AP flours such as Gold Medal, Pillsbury and King Arthur. If you've ever wanted to thicken a liquid such as roux or soup, you'd be much better off using a cake flour than AP because of the higher starch content in the former. Think corn starch for thickening apple pie (and cherry pie?) filling.
Tried it in a curry sauce and it broke down during dinner service, I threw some potato starch in for a quick rescue .