If you are talking about the cradle that goes about mid-way and support the arm they don't work. I have looked at several coaches that have them installed by the factory, and have talked to coach owners, and the water still pools in the middle of the fabric. The advantage to them is they keep the topper tube from bowing from the water weight.
Some folks have made frames out of PVC to support the slide topper and aid in water drainage. Here is the link to an article explaining one solution.
MultiBrief: Finding the solution to slide-out water pooling is a learning experience
Google "slide topper support" and read to your heart's content.