what is it exactly that plato despises about artistic expression?

In response to Tyler Ayres, question about plato and his reason for disliking artistic expression. Plato believed that the brain was limited and unable to grow, so he thought that if you ad in artistic expression then there would be less room in the brain for realistic thoughts and you wouldn't be able to come closer to the truth of the world. Also Plato believed that art would be a pollution of a perfect society and that if art was even to be considered to be aloud inside of a perfect society then you would have to exam each peace to make sure that it wasn't to suggestive and will not pollute the brain in a negative way. My question to you is would you consider graffiti to be a pollution of are walls or to be an artistic expression of our culture.