Pros:
Keeps leaves and debries off the top of the slideout.
Cons:
Will fill up with water when raining
Another awning to keep clean and maintain
More holes in your TT
expensive to add
I thought about getting one as well, but decided against it. Carry a small ladder that will allow you to sweep it off before departing - I use a small electric blower. I think it would be hard to wipe it down once retracted but I have never worried about water on top of it however, I can run my AC when parked at home to help it dry off as moisture inside a closed TT can cause mold/mildew. Would a topper keep the top dry in a rainstorm? Maybe would help, but would it be bone dry? I'm thinking not. Maybe someone else can chime in more on that.