I used a cover once. It only lasted a few months. A big storm came through and tore it up. And yes, it was secured properly. Never again will I waste money one one. We bought our previous trailer new and had it for 10 years and never covered it. The only thing a cover would have helped would have been to keep the decals from fading. Other than that our trailer looked great.
I do cover the tires in the winter more to keep the wheels from rusting than anything else. I also cover the AC in winter.
Where I can see real value in a cover is in extreme climates like the SW or in Canada. There are lots of trailers and motor homes in my neighborhood and in the 28 years we've lived here we've never seen any of them covered. When all is said done, the best thing you can do is inspect and maintain your trailer. Covers are for the most part personal preference. I believe they help in some circumstances but not most.