All experience is delusion. Our senses are imperfect and do not show us reality as it is. Our emotions, likewise, are based on delusion. You acknowledge that many delusions are beneficial but disregard theism as being a useless one. This is simply your perspective on it. It seems a little rude to declare it, generally, to be useless as a whole. It has been shown that when Buddhist monks meditate and when Catholic nuns pray, they are able to reach higher states of consciousness and focus. I imagine that it'd be difficult to make a case that it was "useless" to hold such beliefs, as they literally reached a higher state of focus in the physical world because of them.
Also, I think you look at religion too simply. Your statements presume views, like "one either is or is not a theist." Or "one follows a god or does not." But, I think, these views are subjective and, I believe, farther from the truth.
For example, I lived in Mongolia for a time. I interacted with monks who followed a colorful form of Buddhism. They worshiped the Buddha as something akin to as god and preformed rituals every morning in honour of different deities. But, when I talked to a monk there, he told me that there were no gods or supernatural forces in the way in which we think about them. They are beyond our comprehension. I asked him why they taught meditation techniques that seemed to me to be focused on purposefully deluding the mind. He said that that was exactly what they were for. To delude the mind into seeing something closer to the truth.
In linear thinking, we cannot hope to grasp the nature of reality. Through religious delusion, perhaps we can come closer. The focus and clarity achieved through these types of meditation quiet the "monkey mind" that always bombards you with doubts and distractions that cloud your perception infinitely more.
Does this man believe in a god or not? Is he a theist? Or not? These are blurred lines.
