At stake in this case is nothing less than the essence of a free society. Even more important than the method of selecting the people’s rulers and their successors is the character of the constraints imposed on the Executive by the rule of law. Unconstrained Executive detention for the purpose of investigating and preventing subversive activity is the hallmark of the Star Chamber. Access to counsel for the purpose of protecting the citizen from official mistakes and mistreatment is the hallmark of due process. Executive detention of subversive citizens, like detention of enemy soldiers to keep them off the battlefield, may sometimes be justified to prevent persons from launching or becoming missiles of destruction. It may not, however, be justified by the naked interest in using unlawful procedures to extract information. Incommunicado detention for months on end is such a procedure. Whether the information so procured is more or less reliable than that acquired by more extreme forms of torture is of no consequence. For if this Nation is to remain true to the ideals symbolized by its flag, it must not wield the tools of tyrants even to resist an assault by the forces of tyranny.
—Justice Stevens, quoted in SCOTUSblog
Salon has a good summary.