The only ones I know are newish race boaters that recently bought an Epic, Stellar or Look boat for $$$$$ and think that protecting the finish with a cover will up their race times. Of course a little dirt behind a flapping fabric cover against the finish can be worse than the sun exposure on a roof top.

Fact is there is little if any bug stuff on my boats on top of the truck. Just dirt and dust that rinsing can take care of.

  I would only worry about UV protection if the boat is all day everyday on top of the car.

In my case when I go paddling the driving is mostly on a nice concrete road but the last +/- 500 m are through a "rock-dirt road". If a car is going the opposite way relatively fast some little rock pieces tend to fly, no biggie but they could definitely leave a little scratch on the boat. The same as when little rock pieces fly off a truck.

I don´t worry about it but i have thought about getting a boat sock to reduce this scratching to a minimun.

My opinion on the 3 types of covers:

Danuu Style Regular Cover: This is the one CLC sells, adjustable but not a tight fit, always slapping the boat around and like Grumpy said, get some dirt under it and it will start sanding the boat with this flapping.

A Full Cover: Like these "sunbrella", in theory the best type of cover, a little padding between 2 layers of fabric. Custom made, barely any extrafabric to flap around. I would only get one to throw my boat around instead of gently setting it down when taking it of the car.

The boat sock: stretchy fabric, No IMPACT protection, decent scratch protection, decent UV, Easy in, easy out. However if you dont have a grass patch and slide the sock onto the boat with any sand, it´ll scratch it a bit. I DOUBT there are any for regular kayaks tho.

I´d go

Sock > No Cover > Full Cover > Regular Cover