Ubergolf
There are car covers and car covers. Depending on what level of protection you want.
For all weather outdoors (rain, snow etc) obviously a heavy duty cover is needed. These heavy covers will indeed cause paint scratches, if put on a dirty car.
If its just to keep the sun off for say a day, say whilst at work, then a lightweight cover will suffice.
The lightweight cover will have a less tendency to mark the paint .
I used a lightweight cover on a black car for several years every day as I parked the car at work under trees. The birds of course used to poop on the car as well as the leaves falling on it. The cover worked very well and did not mark the paint work.
With a lightweight cover the wind actually gets under the cover and lifts if away from the paint which is why it needs to be tied down.
The bra film as mentioned previously will yellow with age, as I have had that happen to some film put on a white car, to stop stones flicking off the wheels and taking paint off the car body behind the wheels.