This question has been debated as long as there have been slides and awnings. And it's almost as divisive as the motor-coach, 5-er, TT debate; or the Ford, GM, Ram truck debate; or the wood, charcoal, propane grill debate. Some say they keep slide cooler, protect the slider roof membrane, and keep leaves, pine cones, sticks, acorns, and other debris off your roof. Others say they collect rain water, flap in the wind, need constant adjustment and maintenance . . . and the cost is outrageous!
I've been full-timing for 7 months, now, and have camped in a variety of locations. I prefer camping under trees to keep my trailer cooler; and I just like the ambience of that "camping in the woods" feeling. Yes, I get some junk on the roof, but it really doesn't take long to clean it off. I have a fold-up/telescoping ladder that I haul in the truck bed to use for trailer washing & waxing, window washing, roof maintenance, and the like. I just haul it out, set it up against the slide, and use a broom (or a sponge mop) to sweep it off. Only takes 10 minutes (max) from start to finish - as part of my normal tear-down procedure. I've toyed with the idea of a topper for my slide (which is about 18' long), but can't justify the price.
The couple camping next to me (at my winter home in Texas) have awnings on each of their 4 slides. They have to get their ladder & broom out once a month to sweep birds' nests from under their awnings. Then they have to get out the bucket & sponge to clean the bird poop off the sides of their trailer. Seems to me they have more maintenance to do with the awnings than I do without! That's just the way I see it.