I keep a cover on mine and have since I got it new 5 years ago. I've been very pleased with the results; the trailer basically looks new on the outside. I bought it more to keep the trailer out of the sun than the rain. Most covers aren't waterproof; they are breathable and will keep most of the water off your trailer, but do let some come through. They dry up pretty quick though.
I would stay away from Adco if you're looking to purchase a cover. I went through two of them in three years (I got one replaced under warranty). Both started to deteriorate within a couple months of getting them out of the box. I have a Carver now and while heavier and more expensive, I am very pleased with it. There are no tears, the fit is very nice, and it basically looks brand new two years in. I think the warranty is 5 years on this one.
