Abdulmutallab might know the identities of those who he trained with. That could be very valuable in stopping similar attacks. He probably knows the location of the training base. He probably knows the identity of those doing the training. However, Abdulmutallab is lawyered up and not talking. Fine you can't use the information that you get from interrogations to prosecute the guy. But can't you still interrogate him to get valuable intelligence information? See this in the third to last paragraph in a Washington Post
Abdulmutallab remains in a Detroit area prison and, after initial debriefings by the FBI, has restricted his cooperation since securing a defense attorney, according to federal officials. Authorities are holding out hope that he will change his mind and cooperate with the probe, the officials said. . . .