I agree with you Bill about the oil, although I'm not sure if thats thr driving force or a secondary "benefit".
As for the intelligence, if its highly placed, which it obviously will be if it exists, its going to be immensely valuable during the invasion, and therefore they will want to keep it in place as long as possible.
If you've got someone in Saddams inner circle wouldn't you want to keep him there whilst you wage war against Iraq?