It is important to me that the US finds WMDs because otherwise IMO the war was purely an offensive attack and not defensive. I never bought into the whole 'regime change' argument. While it may alleviate some people's concerns about the war by providing a rationale ex post facto, it just doesn't make sense. Given the heightened tensions of terrorism before the war and the Muslim population of Iraq, why would the US start a war there out of humanitarian concern? Not only would you stir up a hornet's nest of possible new al-Qaida groups, endangering the lives of Americans and raising the costs of national security, but you also risk lives of U.S. military and spend billions of dollars - and all because you are concerned about the plight of the Iraqi people? Doesn't sound like conventional foreign policy to me, especially with GWB. How many despots have we dealt with in the past in South America, Africa, and Southeast Asia who have murdered millions of their own people, and we only learned about after the fact and did nothing to stop them? That's why WMD is the stronger motive, because it actually makes sense to make all those sacrifices and go to war if there is an imminent threat of a massive attack.
Some people are quick to dismiss a 3rd possible motive (oil), but after one incident I'm not so sure. After the U.S. declared the war was over in Iraq, Syria began to block access its oil pipelines coming from Iraq (to reach the Mediterranean Sea). This was reported only briefly in the news, and not on any of the major networks. The next day GWB began to hint that Syria was harboring terrorists and carrying WMDs, and the administration began to hint at a war against Syria (which it later back down from). There was nothing reported to connect the two events together, so of course it might be coincidental. But if the US doesn't find the WMDs then I'm much more inclined to believe 'oil' than 'regime change'.