In Witchcraft, anger is always seen as a waste of time and energy that could have been spent in better ways. Most often witches are painted as negative soul stealing (insert expletive here). However, the Law of Three and the borrowed idea of Karma, has always shown that any anger spent is anger wasted on no one but the person who is angry. To think that a god, any god, would really care if you were angry with it goes against the notion of a being that knows, sees, does all. It reminds me of a child throwing a tantrum because the parent told him/her to think for themselves, or do something themselves; the child may throw their anger at the parent, yet it is the child that suffers from their anger far more than their parent (as the anger will soon subside, and the child still has to take out the trash).
Similarly, the anger, being negative and repulsive, will never allow the angry person to receive, or attract, what they are desperately seeking. However, as many witches, wiccans, and pagans have come to learn, anger does well to make them feel better for that one moment of pure hate; but when the ball drops, it drops into their lap and they have to deal with it - not anyone else, nor any deity.
That is dangerously close to voiding righteous indignation as anger, which DOES have value. I am getting the idea from reading this that you mean all anger should be avoided, which is not using the law of three. Please correct me if I am reading this wrong.
I think anger can be useful as a motivating force. If you allow anger to consume you, and 'burn you up', it is useless. But if channeled, it can be an unparalleled source of energy.