I have always believed that the primary reasons we went into Iraq were to lay a public smackdown on a muslim terrorist nation ("pour encourager les autres", if that's the phrase), and to establish major bases right in the middle of the Islamic world. The ones at Qatar and Bahrain are not enough, and the fact that we had to shed blood in Iraq to gain control of the turf makes us more likely that we'll insist upon keeping these bases, rather than, say, those in the above two nations (or Kuwait), if the politics got tricky.
I see it like this:
There's no one overwhelming reason for our presence in Iraq. Here's a breakdown, as I would posit it :
Smackdown in broad daylight/bases 30%
WMDs 20%
Oil flow and control 30%
Other 10%
Saddam's attempt to kill Bush-41 10% (Yes, I think that it did play a part)
Create a democracy in an Arab/muslim Middle Eastern nation ???%
You can assign your own percentages, or introduce other reasons, of course. Do we want the bases to crack down on terrorists or dominate oil flow? Both, it seems.
Regarding the WMD issue, how silly would the US look if we admitted that we were wrong about Saddam having WMDs, and then a whole bunch turned up? I know Saddam is a miscalculating lunatic, but think about this:
Why would Saddam give up the benefits of having WMDs without gaining the benefits of NOT having WMDs? I still think they're in the ground, or in Syria. I can't believe that Saddam unilaterally destroyed them, and then hid that fact.