-- Last Updated: Oct-07-11 11:00 AM EST --
A cockpit cover won't stop UV damage to the plastic. A good combo would be the silver tarp and a mesh cockpit cover for bug control. It also lets the boat dry out. An oval of plastic window screen held on with a bungee works well for this.
I have regular waterproof cockpit covers, but only seem to use them when I have to carry equipment in the boat while it's on the car. Moderate rain doesn't add much mass to the boat, as the cockpit opening is rather small. If you live somewhere that gets torrential downpours, a cover while driving is probably a good idea.