I don't care if they make my washing dry any quicker or not. Everyone should use them because they make washing on a line look so much more lovely. A saggy sad line looks slatternly, but shove it up with a prop and it instantly looks like something from a Ladybird Book childhood.
It even reminds me of my own, actual childhood. My mother was, in fact, a slattern but she did an ace washing line, boosted with an old grey wooden prop which, in my memory, was about 20 feet tall.
YANBU: Yes, props should be mandatory
YABU: Saggy and proud