Miller's argument, ostensibly, is similar to that of my work with John Donohue on legalized abortion and crime. As discussed in Freakonomics, unwanted children are at increased risk for crime, and legalized abortion reduced the number of unwanted children, thus less crime. Miller, however, makes a key mistake in his logic. While it is true there have been many millions of abortions (although according to the official statistics more like 35 million than 45 million), even if those abortions had not occurred, there would not be that many more Americans today. The reason is that the primary impact of an abortion is not to reduce a woman's lifetime number of children born, but rather, to simply shift the timing of a woman's fertility from early in life to later in life. Based on a paper by John Donohue, Jeff Grogger, and I which will be out in a few weeks, I would estimate that each teenage abortion reduces lifetime babies born to the mother by maybe one-tenth of a child, or possibly even less. (For a woman who gets an abortion in her forties, the impact is obviously larger, but there are very few of those type of abortions.)
The moral of our story is that assessing the economic usefulness of hypothetical humans is a terrible way for pro-lifers to argue their cases. If Miller believes that a fetus is a viable human, and that plunging it constitutes murder, he can stop there: He can't predict what millions of people born to reluctant or unready mothers would have done when they grew up.
That said, fingers are crossed that Steve Sailer will offer a take on this.