If you're referring to the slide toppers, yes I certainly believe in having them.
They aren't really designed to keep rain off the top of the slide, although they certainly do perform that function. It's the debris from trees, leaves, sticks, and what-not that they are designed to catch so when you put your slide in, you don't have to either go on the roof to sweep off your slide top, or worry that some debris is going to tear your top slide seal.
Mark, Vicki, & Scout THE dog
2015 Hemisphere 282RK
2010 F250 Super Duty
1992 Goldwing Aspencade