Critics should be clear on whether the administration is abandoning its earlier reasons or not. It's not truly a shifting rationale unless early reasons are abandoned when the situation changes, at which point new reasons suiting the current situation are brought out. On the other hand, if the administration brings out one argument and exposes it to the media, lets it sink in and explores the details, then brings out another reason without disclaiming the previous argument, then the administration is actually building a case on many arguments.
My impression is that tax cuts were justified by a shifting rationale, but the war on Iraq is being justified by accumulating several independent reasons to attack. I don't claim that the sum of arguments is sufficient, I just claim that's what the administration is trying to do.
It's an odd facet of human nature that presenting multiple independent arguments can be a bad tactic. If one of the arguments is perceived as weak, opponents seize on that argument. Then the other arguments are ignored, as if they depended on the weak argument, and it becomes very hard to convince that person. However to a rational listener, a weak argument should only cause the stronger arguments to be thrown away if they depended on the weak argument, as in a chain of causes.