"Is torture immoral?"
Is deliberately hurting someone a bad thing? Yes. So is killing someone. There are situations where killing someone is moral, convicted murderers and enemy soldiers topping the list. So I think it's fair to say that some situations may justify torture.
"What is torture?"
There's lots of disagreement on this. I haven't seen anyone claim that branding or pulling out fingernails isn't torture, so there is some agreement. There are also people who claim regular prison sentences are "torture" because of the mental suffering they inflict, fortunately very few or we'd have a tough time dealing with common criminals. "Waterboarding" (temporary suffocation while having water run over your face) is described as torture by lots of people even though it involves no physical damage or even pain (as opposed to mental suffering).
Various activists have been trying to broaden the definition of torture with the perverse result that public support may be increasing. Personally if wrapping someone in an Israeli flag makes him reveal useful information, I'm all for it. And if legalizing that means allowing much worse things, I consider it a necessary price.
"Does performing torture corrupt our people?"
I'd expect so. Killing people isn't helping the moral development of the combat troops. Police officers can have an even rougher time with killing because they operate in a more ambiguous environment. We deal with this by setting clear rules of engagement, supervising the guys on the line, and taking action to deal with those who cross the line, including criminal prosecution when called for. I don't see torture as different in that regard.
"Does torture actually obtain useful information?"
Certainly not when being used to punish a prisoner or entertain the torturers. As part of a controlled interrogation there's a long history of success. I'd expect other methods get better results in most cases. But that's something the local supervisors will have better knowledge of than the homefront debaters. And even if something doesn't work well that's no reason not to let it be part of the toolbox.
"Is torture a useful threat?"
A terrorist under interrogation is going to have a much rough time standing up to something if he thinks it is one in an infinite series of techniques to be brought to bear. If he knows it's the harshest measure allowed by law and after that he's home free he'll muster up the determination to last that out.
Waterboarding probably isn't a threat any more, though. After all the publicity it's gotten I'd expect terrorist training camps are setting up multiple shower stalls with cots and rolls of saran wrap. The next wave of captives are going to be used to dealing with it.
"Do vague laws handicap operations in the field?"
Hell yes. If you want to stay out of jail you stay clear of crossing the line. The fuzzier the line is the farther away you have to stay to be sure you don't cross it. So legal options will go unused.
I've got this problem on my day job with the ITAR regulations. The rules are written by lawyers and implemented by engineers so we deal with fuzziness by declaring information non-releasable (thus avoiding all risk of jail). This is screwing up operations enough to make Britain threaten to quit the program. And I don't have to worry about a Senator calling press conferences to denounce my mistakes.
"Where should the responsibility for drawing the line be?"
John McCain thinks it should be in the chain of command of the interrogator. As he puts it, "You do what you have to do." But I haven't heard him say he'd be testifying at a court martial to declare that he intended his law to be broken in that particular situation. That strikes me as a clear abdication of responsibility. If Congress is going to pass a law it should be one they intend to enforce and are prepared to accept the consequences of. Not shrug it off with "well, we didn't mean you should avoid torture in that situation."
"Can we win the war without using torture?"
Yes. At a cost of more deaths on both sides. The less flexibility we have the more force we'll have to use to defeat the enemy. But I presume this is acceptable to those who consider torture worse than killing. I'd rather minimize the body count for both sides.