In reply to GrahamD:
Yes the benefit is only cosmetic. Older carbon frames were painted with thick brittle paint, which chipped easily. I had a 2006 Roubaix and I took a small piece of paint along to Boots in the hope of finding a matching nail varnish; the assistant there succeeded in making it clear she didn't believe my story and thought I was a trannie looking for something nice for the weekend. That thick brittle paint sometimes cracks in areas where the carbon frame flexes, giving rise to worried posts on internet cycling chat sites.
I tried to blob in the nail varnish neatly with several coats to fill the gap left by the flaked off paint but it wasn't really successful. Some bike manufacturers do supply touch-up paint.