Proponents of the a national missile defense (going back to Ronald Reagan) suggest that there is a moral obligation to build it: that to leave citizens defenseless against attack is an immoral policy. In addition, the threat of massive retaliation against a civilian population as a deterrent, while a longstanding pillar of U.S. policy, is viewed as immoral.
Interesting to note that the Rumsfeld Report, which surveyed the likelihood of attack from ballistic missile in the future, represents a dramatic change in U.S. foreign policy thinking: it suggests that rogue states developing missile capability are a real threat to national security.* And while there are advances in technology that will allow the proliferation of these weapons to rogue states, the U.S. capability to massively retalitate has not changed. In other words, moral or not, the conditions for deterrence are still in place. The thinking of Rumsfeld et al proposes that leaders such as Saddam Hussein or Kim Jong-Il are irrational actors-- willing to risk their country's annihilation for the sake of damaging the United States.
Pre July 15, 1998: assume all world leaders are rational.
Post July 15, 1998: assume despots are irrational.
* This was the third report commissioned by the newly elected conservative 1994 Republican Congress as part of its "Contract with America." The 1994 Republican Congress, asked U.S. intelligence agencies to assess the need for missile defense. The 1995 National Intelligence Estimate concluded "no country, other than the major declared nuclear powers, will develop or otherwise acquire a ballistic missile in the next 15 years that could threaten the contiguous 48 states or Canada." This was a setback for missile defense. The Congress set up another commission to investigate whether the 1995 NIE was compromised by politics (i.e., Democrats opposed to missile defense). This new independent commission, chaired by former CIA Director Richard Gates, announced their conclusions in December 1996: "For sound technical reasons, the United States is unlikely to face an indigenously developed and tested intercontinental ballistic missile threat from the Third World before 2010." In addition, "There was no breach of the integrity of the intelligence process. Beyond this, the panel believes that unsubstantiated allegations challenging the integrity of intelligence community analysts by those who simply disagree with their conclusions, including by members of Congress, are irresponsible." Missile defense proponents in Congress respond by commissioning yet another panel, with Donald Rumsfeld as the chair. And its conclusions re-defined the ball game. Rather than answer the question of whether, when, and will rogue nations develop ICBM capability, it concludes that the mere possibility of developing ICBM capability threatens the U.S. Finally! A justification for missle defense!)
Sources:
Jones, Sherry. "Missile Defense." Frontline. Azimuth Media, Washington Media Associates, WGBH Educational Foundation. 2002.