Just remember that any part of the paint that comes in contact with a cover will be dulled just from the cover moving as the wind pushes it around. But then if you are parked outside then the cover may be less damaging than the sunlight.
My point is simply this. The reason a new car's paint gets dull and old looking is from the things that touch it. It takes a long time but every micro scratch will cause this. Over the years they add up and the finish looks dull. So why speed that process up.
I remember the nose bras that were popular in the 80s. I worked at a Ferrari dealer and I have seen the paint under the 'protective nose bra' rubbed dull and in need of a repaint.
Just my .02
