So I haven't read this because there's all this other stuff higher in my queue that's based on the assumption that he's wrong and life is good, but David Benatar wrote "Better Never to Have Been: The Harm of Coming into Existence" claiming, roughly, that because humans seem to innately experience a loss of X as three times as bad as a gain of X, it would be better on average for a person not to be brought into existence.
From the reviews it seems Benatar's practical upshot is that people shouldn't kill themselves right away, nor kill their existing children, but neither should any more children be brought into existence.
If anyone is really into this topic it would be awesome for someone to read through the book and write a review that assumes the audience understands cognitive biases, astronomical waste, and the coming possibility of revising human nature so that "human nature" arguments aren't acceptable unless they cover "all possible human natures we could eventually edit ourselves into".
Still haven't had time to read the book, and probably never will, but John Danaher has been covering some arguments over the book on his (excellent) blog: Part One, Part Two, Part Three.
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.