First, I'd like to point out that I agree -- there are plenty of other reasons we should have gone to Iraq. The imminent threat wasn't actually there, it was merely a rumor. Fine. But there were people in danger, there was a dangerous government in place, and people were hurting. If we care about people enough, then it's up to us to do something for them. (Which I suppose we did -- a wee bit late.) I'm not asking us to pull troops out, you'll note; there's a lot yet left to do over there.
Before I let it slip though, there's a basic flaw to the above: consider its equivalent in a court of law. You arrest a "bad guy", convict him of a crime, and send him to jail. You later realize that he was wrongly convicted: the evidence, when re-examined, didn't actually add up. Luckily, you don't have to let him out, because other evidence shows the likelihood of another crime -- so we'll just leave him in prison and call it even. Sound fair? No. Maybe in terms of "getting what you deserve", yes, but not in terms of due process. When you screw up, you apologize. If you have to abandon one line of reasoning and pick up another, you do so carefully. Were there other reasons for going to Iraq? Absolutely. And we should have talked them over before staying -- at least to make sure we were clear on why we're there now (considering it's obviously not because of WMD's.)