I'm on the fence. There does seem to be some affect on heat gain, and I've rarely found debris between slide roof and topper. We've yet to have a leaky slideout either, so the topper probably assists in that regard. Several members have installed innovative drain modifications to help keep water from puddling on their toppers, though I've personally never found it to be a big issue.
The downside consists of the noise created when they flap al night in a big wind, esp on the Oregon coast in winter, and I often get up and pull windward slides in. There are various remedies for the flapping, but if the fabric stayed tight, we wouldn't need them; adjusting tension is difficult, or not manufacturer advised. One December in Eden, ID, after waiting out a snowy thanksgiving, I had to get on an icy roof to break loose packed snow and ice on all four toppers before we could pull the slides in to leave - the toppers were jammed. Lastly, the living room topper's fabric once wouldn't roll entirely up and hung out of its roller with the slide closed; a major bolt had fallen out and the part was ridiculously hard to get. That's when I first discovered regrets about owning Carefree awnings and toppers. I'm not alone in that dept.
Joel