The painting rule could be given the name "the phantom rule" it was specifically designed to stop the Phantom. As some details of the new production Phantom became clear it was detailed that the new boat would be painted in an epoxy paint which gives a very high quality finish and in the matt style it looks unreal too.
A scare campaign went through the WC claiming that painted boats would have a considerable advantage. They declined to back the claims up with anything like facts.... it reminds me of this classic from Will Ferrel global warming
Paint is just a better solution, as mentioned above, its easier to repair, lasts longer and is lighter than gelcoat. There is no speed advantage at all so there is no justifiable reason to ban it.
But the same could be said for the 2 types of cloth in a mainsail rule too, every independent sailmaker in the class agrees that it should be allowed and it increases the life of the sails and makes them cheaper. So again, why would you ban it?