This topic will be around as long as there are RV's. (I own a Jayco Pinnacle.) COVER YOUR RIG UP!! My opinion comes from someone who lives in the Northeast. When all the snow lies on the roof and then starts to melt, several inches of heavy slush lays on the roof. All it would take is a very small area to leak and destroy most any part of the unit. The ultimate concern is usually having to spend money, sometimes as much as $500+ for the cover, for largest size/quality, etc. Why would anyone balk at spending a little bit more to protect an investment that costs thousands more than that, and that you will enjoy for many years?
Let me put it another way. If a free cover came with your trailer you'd cover it, correct? Well there's your answer! Another explanation or defense you hear all the time is that "..the dealers don't cover theirs." The reason for that is simple. It would be cost prohibitive to cover all of their inventory. Also, they are dealing with brand new vehicles and will have them sold by the time they have any issues with them. Used vehicles they cross their fingers and try to sell them as quickly as possible. Again, they don't "own" them.
Lastly, don't cover it with a blue tarp from Home Depot! You will look like (forgive me) "trailer trash" and it will tear! Most all of the designed covers are extremely light weight with a reinforced roof and breathable sides. Take care of your unit and it will in turn take care of you and your family.