I think you're getting at the question of freedom. I agree that we have an open future and that our decisions do make a difference. It's also true that the past has a strong hold, and often we don't really choose our future even if it may appear that way.

I recently (re)discovered Buddhist meditation and I’ve had this topic on my mind for some time. As others have noted, the question as framed is already leaning in favour of usefulness and thought. We could question both as starting points. But if we take it as given, I also think some pragmatic, non-dogmatic mixture of optimism and pessimism is probably best for functioning practically in the world. We need to be mindful of negative possibilities, or we’ll be caught unawares by dangers. But we need to have some faith or hope that things will ultimately turn out well. Otherwise, it’s hard to see how we can escape paralysis of the will, inability to act out of sheer despair. Schopenhauer advocates extinguishing the will as a result of his metaphysical pessimism, and I think he’s consistent in that. It’s also interesting that Schopenhauer was from a well-to-do family and didn’t have to work for a living, unlike Kant or Hegel for example. I wonder if he would’ve reached different a view if he had had to lead a more engaged (and dependent) life out of necessity.

I want to take this question beyond the criterion of usefulness in the narrow sense though. I would argue that optimism and pessimism are valuable experiences in themselves, apart from any benefit or use. Each one expresses a fundamental truth about human existence. I would even say that a human life is incomplete if it hasn’t gone through both extremes of the spectrum and ‘conquered’ them in some fashion. Each extreme is metaphysical in its own way and opens the door to an experience that transcends ordinary reality. I agree with the folks who said that the key experience on the side of pessimism is anticipation of death (or Thanatos). One way I’ve found to reach this experience is through the Buddhist nine-point death meditation. Going through the nine points, one gradually feels the finitude of being a mortal, that uncanny feeling of the existentialists that expresses our homelessness and alienation in the world. Freud thought that the unconscious doesn’t have a concept of time, and it seems animals don’t have the experience of finitude in the way humans do. So this experience probably requires use of conceptual thought and the reality principle. I’ve heard that Buddhist monks meditate on death daily, as do Christian ones in another way. That’s not surprising, since it seems we’re wired to forget and evade thoughts of death and anything that might remind us of it, like pain, illness, trauma, failure and negative outcomes.

Eros (or life) to me is the oceanic feeling that Freud talks about, a kind of metaphysical union where boundaries dissolve and finitude is no longer felt. I think he’s right that we feel this whenever we join other living beings in larger units, like in family or religion. I think some such experience of union is necessary to hold an authentic optimistic view. The set of beliefs must have a real counterpart, so to speak. Otherwise they are just words empty of content. There must be some reason posited for why things will turn out well, and to my mind this is usually Providence. The same is true on the other side. I don’t think a person can be a pessimist without direct experience of the negative aspects of life, and ultimately death in some form. So, I would challenge not just usefulness but also thinking as the criterion in the original question. I would say that beyond the thoughts and positions that we consciously adopt we have intuitions of reality. The thoughts spring out of these intuitions, so to speak, and the only way to sustain the thoughts in the long run is to nurture the underlying intuitions.