I have one, and I don't use it. There are issues with it rubbing in the wind and leaving marks. They are also a royal pain in the rear to get off and on. Now if I lived someplace where it snowed and I was putting the motorhome away for the winter and didn't have indoor storage, I may use one. But in California we don't put our motorhomes away for the winter and taking the cover off and on was a pain. I found myself not taking the motorhome out for a weekend because I knew how much of a production it was to get the cover off and on.
But on another note, leaving the motorhome out in the weather it's entire life isn't good for it either.