US did about 10 mostly unpainted a year or so ago because of an accident during the stripping process of the old paint so new paint couldn't be put on for a while (N573US, I believe). Rather than just have one, they did 10 and issued a press release on that.

After the examination period for the skin, they went back to painting all of the fuselage.

I've also been told that the paint, when properly applied, can reduce corrosion on the outer skin of the plane and acts as another layer of protection. That would be an argument for adding it, even if it adds some weight.

Then again, we used to paint the fuel tank of the space shuttle to make it all look nice and realized that was an extra 600 pounds of weight that wasn't needed.