For me to answer that question, you'll have to tell me where you are in the process. If you're talking about a bunch of people who are charged with conspiracy to commit a terrorist act, I don't see why you're going to need it. Presumably you would have the evidence before you arrest them.
I haven't seen a circumstance in which the investigative hearing would help. As Professor Forcese says, you are revealing your investigation as soon as you grab somebody. Presumably, everybody who might have been involved is going to either take off or stop—probably both.