My Outback is the first camper I've had with slide outs. It does not have a slide topper. I am careful when closing up the slides to brush off anything that falls on it. Yea, it takes a little effort to get a ladder and a long handled brush (I use a painter's extension handle and either the soft brush I use to wash the camper with or attach a squeegee to pull the standing water off. Would I like to have an awning topper? I don't think so.
I can't say if they would help or not. From the argument the above poster made, if leaves and pine needles get trapped in the roller and the cover has to be cleaned off before retracting the slide anyway, I can't see how they would be of any benefit. The types that just have rollers, like the awning over the door, and stuff fell off as it rolled up, that might be OK. But I have found out that if my awning over the door is wet with dew or from rain, stuff stick to it and it has to be brushed off or it get rolled up. Now, not only does stuck leaves, but they get rolled up making both the top and the bottom side dirty. If this happens with awnings toppers over the slides, I can't see where they offer very much advantage.
In snow, I think they would be helpful, if the snow fell off. But in a good snow storm, the snow will pack under the awning too. When closing up, the snow would have to be pushed out from under the awning too. I can't see where that would be any easier, actually, it would be harder.
Anyway, I'm not putting them down, it's just after owning a camper with sides, I really can't see where they are much of an advantage. They are also another part of the camper that needs to be kept clean. How do you clean them from the underside when they turn black with mold and mildew?
We're camping right this moment at Versaills State Park, near Versaills, Indiana. As I've looked around the camp ground, campground A, B, and C, I don't see any campers with toppers over their awnings. I'm just saying? It's still a matter of what YOU want.