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.
Yes, but the same could be said of children and the elderly (that they are currently not very valuable to society), but it is still illegal to murder them.
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.