I use a cover on my vega as it came with it fits so nice. I have a cover for my nelo but i never use it. None of my other skis had cover and I have never had any damage on the road other than streaks of bug guts. I have seen skis damaged by covers as a cover flapping in 65 mph wind is abrasive over time. Some covers have mesh to allow stretch and egress of water, but that can also allow in dirt that gets trapped and is abrasive. To each there own on this.
In some way the boat i enjoy the most is a dirt cheap third hand south african fiberglass thing with white gelcoat and some cosmetic imperfections. I just enjoy that boat and never worry about damaging it. And the amazing thing is that i never do damage it. Leads me to think that my worry about damaging my fancy skis is misplaced and not actually adding any magical protection.
I am not a total nihilist. I have seem way too many skis damaged by racks, straps, and poor tie down jobs. I think when it comes to protecting a ski that is a more useful place to focus my effort.
EDIT - i almost drive gravel roads with a ski on the roof!
Current Skis: Nelo Vanquish AIR, Epic V10g4, NK 670 double, NK exrcize, Carbonology Feather, Think Jet, Knysna Sonic X
Former Skis: Epic v10g3, Kai Waa Vega, Epic V12 g2, Epic V12 g1, Epic v10 double, Nelo 550 g2, Fenn Elite S, Custom Kayaks Synergy