This should help solve the problem. A friend said that these can actually be detrimental to a car's paint finish because thy can trap moisture. Other people have told me that the covers are useful in protecting the paint from rock chips. The real story is that both claims have merit. The purpose of these protective covers is to protect the paint from rocks, bugs and other debris that can damage the finish.
Some people feel that the covers also add aesthetic value to the car.
The disadvantages are that they sometimes fit poorly, which can cause them to flap when driving at freeway speeds, and removing and replacing them when necessary can be inconvenient as well. In addition, and as your friend mentioned, the covers tend to hold moisture, when can result in a faded paint finish.