In this chapter, Diamond never explicitly states his views on how useful religion is. Therefore, we have to infer what he thinks. Basically, he seems to think that religion is a mixed bag for people in a society.
On the good side, religion can keep people from killing one another. Diamond mentions the Fayu, who cannot live in groups who are unrelated to one another without engaging in violence. Diamond says (page 278 in the paperback edition) that religion can give people a way of avoiding this.
On the bad side, religion gives people a reason to die. Diamond says that religion helps to persuade people to risk their lives for their country. He says that religion makes it much more likely that people will be willing to fight and die in battle to defend their country and/or religion.
Thus, religion is of mixed benefit to the average person in society.