The point has been made in the context of the "Underpants Bomber's" cooperation, that under the legal regime preferred by elements of the GOP, Abdulmutallab's family members never would have worked with FBI interrogators to encourage further cooperation from him. Not only does this example vindicate the use of the established legal framework for interrogating (and prosecuting) terrorists, but it casts that framework in a virtuous light. Or so goes the argument.
What revelations about Najibullah Zazi's case shows is that there is also a shrewd, almost cynical calculus favoring the use of the instruments granted by our criminal justice system: the law is incredibly coercive. As the Washington Post details, Zazi's cooperation began "after authorities charged his Afghan-born father with crimes and threatened to charge his mother with immigration offenses -- options that are not available in the military justice system."
Conservatives enjoy drawing a dichotomy that casts military commissions as strong and traditional legal methods as weak. But that view isn't really borne out by reality. The law extends far. Very far. If interrogators or prosecutors decide, there are all kinds of options that they can deploy to reach deep into the life of a suspect, options which will sometimes invite, but more often compel cooperation.
Military commissions or indefinite detention, as the Post points out, are a different story. They create a legal vacuum where these options simply aren't available for the most part. That's important to remember for when conservatives attempt to justify the use of these methods on the basis of interrogation and intelligence gathering. In significant respects, the decision to work within the established criminal justice system makes FBI and Justice Department officials less, not more constrained in terms of the methods they use to gain cooperation from terrorists.