The problem is evidence obtained by torture in inadmissible. From John Kiriakou at responsiblestatecraft.org:
The Guardian has reported that a recently-declassified CIA Inspector General’s report from 2008 found that CIA officers at a covert detention site in Afghanistan used a prisoner, Ammar al-Baluch, as a “training prop,” taking turns smashing his head against a plywood wall and leaving him with permanent brain damage.
Baluch is currently one of five defendants before a military tribunal at the U.S. military prison at Guantanamo charged with participating in the planning for the September 11 attacks. The case has been stuck in the pre-trial phase for 10 years, in part because much of the information that the government wants to use against the defendants was collected using torture.
In addition to the five, Mohammed al-Qahtani, known as the potential “20th hijacker,” is to be transferred so he can be treated at a rehabilitation and mental health care in his home country of Saudi Arabia. Al-Qahtani was reportedly suffering from mental illness and brain injuries as a young man, which were exacerbated by torture after he was captured in Afghanistan in 2001, and was not deemed fit for trial.