Let me turn your question around. If your utility function puts value in the mere existence of people, regardless of how they interact with the larger world, doesn't that mean having babies is as wonderful as killing people is terrible? Is somebody with 12 kids a hero?
Is somebody with 12 kids a hero?
Or a serial killer with a large family? "Sure he might have killed 3 people -- but he's a father of 5!"
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.