In general, people are much more valuable to society alive than dead. This does not apply to unwanted babies.
Why does the fact that the parent does not want the baby imply that it has little value to society? I have a friend that was adopted (which seems to be the main alternative to abortion) that I value very much. If he was aborted as a fetus, then I would (probably) be much worse off than I am. Are him and I (and everyone he engages in mutually beneficial exchange with) not a part of society that is thereby gaining value by his non-death.
Well, sure he's valuable NOW, but that's after years and years of investment. It's not that being unwanted takes a baby from super valuable to negative, babies are just not very valuable regardless.
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.