The purpose of a writ of habeas corpus is to cause a government to release a prisoner or show through due process why the prisoner should be held. Of Guantanamo's approximately 270 detainees, many certainly are dangerous "enemy combatants." Some probably are not. None will be released by the court's decision, which does not even guarantee a right to a hearing. Rather, it guarantees only a right to request a hearing. Courts retain considerable discretion regarding such requests.
As such, the Supreme Court's ruling only begins marking a boundary against government's otherwise boundless power to detain people indefinitely, treating Guantanamo as (in Barack Obama's characterization) "a legal black hole." And public habeas hearings might benefit the Bush administration by reminding Americans how bad its worst enemies are.