Paul doesn't tell us not to get angry. There are plenty of things worth getting angry about. I was reading an article the other day about the number of people in abject poverty due to violent conflicts in Africa that have persisted for years. It was heartbreaking... and worth getting angry about. We should get angry about suffering and injustice in the world.
What Paul warns us about is letting anger linger in our souls, because it starts to change us. If we let the anger be the voice that persists over years, it can drown out every other voice. Paul is telling us to watch out for anger, to deal with it quickly, and therefore to ensure that anger doesn't remain the guiding force in our lives. Let the things in the world that make us angry help us decide how to act, but then we can set aside the anger and move forward in love.