Regardless of the administration's wisdom in making these statements, do you really think that the terrorists are evaluating what weapons to use based on our official position?
I think that they have adequately demonstrated that they will act utterly irrationally (from our point of view) and use any weapon at their disposal in the most devastating way that they can.
Now the conclusion has to be reached that both sides are insane or rational. Which is it? Escalating the use of WMD by public statements does not help. I imagine that if FDR had pronounced that we would use poison gas against Germany during WW II if we had to, the Germans would have used it.
Why encourage more violence instead of less by public statements? We have had enough problems in the world trying to contain nuclear weapons without now advocating their use.