I think that the hypothetical situation is illogical in practice. How could you be definitively certain that the person had information that could prevent a terrorist attack unless you knew what that information would entail? The key element in most terrorism is surprise. To posit certainty that an individual has information that could prevent an attack presupposes anticipatory knowledge of the attack itself, thereby already crippling the attack.
Although I think that it makes an interesting thought exercise, I don't think that the situation is feasible.
What you are doing is trying to disprove that there are categorical moral imperatives. You can always find a grey area in some idealized scheme that you fine tune to a certain set of preconditions.
You can defend torture, you can defend torturing children, you can defend mass murder, you can defend wiping out entire cities at once and you can even defend genocide all on perfectly sound moral logic. What's more you could even justify it by using some arbitrary "scientific" measuring stick like utilitarian calculus over a pre-decided temporal interval and a certain degree of secondary consequences.
That is if you keep strictly to the scheme and divorce yourself from reality completely. Which is not something you should even want to do.
One (wo)man's terrorist is another (wo)man's freedom fighter. That I think is a non-trivial case of moral relativism that is not so easy to dismiss. Especially when you put it in a historical perspective, but let me not succumb to Godwin's law already.
There are many pragmatic problems with torture and some are by design. For example getting false information can be a goal, instead of an unwanted consequence of torture. It can be politically useful to get some "intelligence" as proof or confirmation of some preconceived assessments. Bribes, black mail and other instruments on the political repertoire can be used to further that goal, like a justification for wars, bombing campaigns or extra-judicial executions by UAV or something of the sort. (In which case it can be considered wise to prepare some after the fact justification when things go terribly wrong.) Unintended consequences of torture are perhaps illustrated to some effect by the biography of Ayman Al Zawahiri in whose case torture might not have worked out all that well in a utilitarian scheme of things.
But while you cannot make a categorical statement about (or decision on) torture, which admittedly is an extremely unpleasant and uneasy thing to realize, the objections, logical, pragmatical, emotional etc. are so overwhelming that you should not even take it into consideration, legally or illegally. And conjuring up schemes whereby you would consider it belong to the intellectual domain of Hollywood script writers.
Torture? Yeah right!
Back in the Catholic Inquisition times, religious authorities had successfully extracted the truth from their confessioners ( got the sarcasm? ). So, if you say that torture is a good way of having people to tell the truth, I won't blame the Catholic church anymore for killing millions of people who confessed witchcraft. When pain is being inflicted, you confess whatever the torturer wants so you can scape from the inflicted pain.
The issue with this one is always the shadow of a doubt that your information about the individual is wrong. Regardless of how cast iron you think your information about him is, there is always a chance that YOUR information is wrong. The guy might even think that his information would prevent some egregious act of terrorism, but HIS information might be wrong. I find it difficult to justify torture...ever.
I think it's the second time someone says that it's a waste of personnel. Come on, how many people does it take to torture one guy? (not a joke)
Sure, you need a lot of guys to catch him, but after that, you don't need more than let's say 3-4 guys. And I think they do have people in charge of interrogation. So, they don't waste their time they're just doing their jobs.
Of course, they have a great chance of getting inaccurate information, but if you really think about it, it's not really a waste of time and personnel.
Getting false information or false confessions can be a goal of torture. Getting false intelligence is contingent upon political aims and false confessions together with the admission of secret evidence allows you to bypass law to get rid of certain people burdened with uneasy facts, not suitable for publicity, and/ or create necessary illusions of safety, decisiveness, effectiveness.
As was claimed here before the Spanish Inquisitors were not torturing people to get to the truth of the matter, they already decided upon the truth and just wanted a matching confession. Well it seems from what we know under the Bush regime torture was mainly used for (the Obama regime is much more secretive) it is not an area of great innovation in this regard.
You are right that torture is known among intelligent experts to be about the worst method conceivable to retreive reliable information. (Another one, only slightly more reliable is throwing money around, thinking you are going to buy reliable actionable intel in no need of any verification or further checking procedures instead of local gossip.)
But I do hope, mainly for your sake, that I'm not mistaken if am going to assume that it is not so that your entire objection to torture would be that it's not effective and costs too much manpower.