Why would they be slim? Why would a member of a cell not be privy to the actions of his cell? It's not as if they catch any enemy combatant in afghanistan and assume he knows the next plot in america. What happens when they catch Zarqawi's informant, courier, etc.. how are you going to disintegrate into silly skepticism.
Look, there are chances that we might throw the wrong man in jail, that doesn't mean you must abolish all laws and enforcement.
The issue is not about probability and percentages, the issue is whether One has the right to torture a member of a cell who is involved in a plot to bomb a city, i'm pretty sure that's not a fantasy and it occurs more often than not.