I have found a cover is useful. It helps keep the rig clean and protects it from sun damage.
Just make sure the RV is clean before installing and the cover is snug. Where I live we get a lot of winter wind, I've never had any damage from the wind causing the cover to rub the paint.
No, car dealers do not cover their inventory. But, many people buy a new car/truck, take it home and keep it in a garage.
It just makes sense to take care of your expensive purchases.
If you had a large garage would you leave the RV parked outside?