Yglesias points out this report from the Center for American Progress on sea-based ballistic missile defense, in which Andrew Grotto and Rebecca Grant argue for continued funding of research and development into AEGIS based ballistic missile defense systems. I can't really disagree with that conclusion, but I do think that they overstate their case; sea-based ballistic missile defense is nice on the margins, but except in a very few tactical situations, it is unlikely to change the course of any imaginable conflict.
On the plus side, sea-based ballistic missile defense avoids certain of the problems of land based systems. Land based systems (such as those being installed in Poland and the Czech Republic) carry with them significant political costs, and are vulnerable to political shifts in the host countries. In their current configuration, land based systems are also dependent on a certain constellation of threat. If, for example, we spend a tremendous amount of money building an anti-Iran system in Poland, then the Iranian government collapses and is replaced by a friendly regime the next day, we have wasted a lot of money (this assumes the entire point of the system isn't to antagonize Russia, which may not be reasonable). A ship-borne missile system can just steam to wherever the threat is, be it near Taiwan, North Korea, Iran, Venezuela, or the target du jour.
