The apostle Paul, in this context, is speaking to Christians who are working on a maturity in Christ. It is true that Christians need to learn basics of what it means to live a lifestyle that is worthy of Christ. Yet, it is another issue all together to impliment a mature lifestyle change in Christ.
Does a child behave perfectly in all areas of his/her life. No. Is the training to a mature adulthood ongoing or immediate. Definately ongoing. Living for Christ is an ongoing process as well.
Anger, innapropriately utilized, is a facet of life that needs to be controlled by our understanding of Christ. Is anger something that should be festered. No. Anger should be dealt with and talked about. People should share their pain and suffering. Anger which is not dealt with becomes destructive and without cure. But Jesus wants us to mature in how we deal with the hurts and pains in our life. Why? It is a matter of wholeness. How can you be whole when you continue in unresolved anger?
It is a hard heart that remains in anger. It is a godly heart that moves on toward peace. "Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice. Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you" (Ephesians 4:31-32).