I have been using the more expensive covers 24/7 for over 5 years.
Not caused any problems to paint that I can see (though I admit I am not dead fussy, as main purpose for me is to use car).
They are the fleecy lined ones and have a bit of weight, so don't flap about.
I think, as a lot of our cars aren't always 100% watertight (door/boot/smaller seals etc + other nooks and crannies, where water will settle), then, using a cover can be very beneficial.
My cars left on the street and I think a cover minimizes vandalism, also UV light damage to rubber.
I always keep an old cover that I use for periods of dry/very sunny weather/light rain, and my newest cover should therefore last a lot longer, as I think it's UV light that deteriorates them the most.
Dave