My $0.02...
I use custom weathershield car cover on my car.
I put the cover on even when the car
is a little bit dusty. I am sure I am getting marring from
the cover. However, I think the benefits outweigh the
negatives.
First, I am not a terribly good at washing or QDing my car
(unlike Mr Accumulator - our residential car wash guru here

). Without the car cover, I am constantly washing
and QDing my car and thus introducing a lot of swirls in
the process.
Also, where I live, in the winter/spring, we often get brief
showers followed by bright sun. Those wonderful bead
of waters instantly turned into pollen-ingrained water spots.
Horrible!
Lastly, I have black leather interior. It is an inferno inside the
car without the car cover.