When a group of people become fanatically certain about anything, it seems as if their certainty somehow subconsciously endorses emotional and physical violence to reinforce that certainty. Religious belief or the lack thereof are not very different in this respect. I see no fundamental difference between the violence of the crusades and the deaths caused by totalitarian communism. They are perhaps mirror images of each other- justified in the minds of those who did them either in the context of belief in God or the lack of belief in God although in each case a belief or lack of belief in God wasn't enough to generate the violence. The main difference between belief and non belief is that believers tend to organize themselves into groups, whereas much of the time atheists organize like a herd of cats. People who organize like a herd of cats aren't likely to participate in organized violence
In the USA, atheists and agnostics form a clear minority, liable to be targeted by the violence (physical, emotional, etc) of religious certainty. While this is a depressing thought, I believe it should be taken as a challenge in searching out how we can best love everyone. It has been said that a society can be judged by how it behaves towards those it dislikes. In a society where atheism is a minority and somewhat estranged from the dominant culture, we are morally obligated to show hospitality and social accommodation and can judge ourselves wanting if we fail to do so.
But there is a more pressing way in which I think society and religion need atheists. Le Guin, writing in the novel The Telling has a character state, speaking of fanaticism, "Belief is a wound that is healed by knowledge." Certainty creates mental and moral blind spots which wound our ability to care about other people at all if they don't fall within the "morally approved" categories. Atheism is in a powerful position to call out believers when their ideas are ridiculous or unethical. If religions can't grow and accept when they are wrong, they become morally stagnant and are a barrier between the people who believe in them and God- both a barrier to understanding and a barrier to moral growth.
But what about the souls of those who don't believe? If God is as full of love and forgiveness as I believe I have experienced, I don't believe God's redemptive power to heal people's soul's somehow has an expiration date at death. I once heard my Bishop say that who knows if God doesn't work anonymously through other cultures and religions. I'd say that when the world is morally advanced by atheism providing a strong moral counter point to religious fanaticism that maybe we should be looking to the face and hand of God being revealed in those that don't believe. If God was so offended by people missing the point of their existence, God would have made things more clear. On the flip side, I believe that religion and Christianity as a whole are just as capable of having missed the point about any number of things incorrectly assumed to be basic to their entire sense of purpose and theology. It is more important to be seeking a path to further enlightenment and moral advancement than it is to be battling over who is right or who gets control.
So as both a reminder of needing to love those who we disagree with and as a fresh moral perspective that can lead us closer to each other and to God, I believe religion and society needs atheists.