Some reasons not to tell the truth (the whole truth anyway):
The crushing impact on US morale, resulting from the public comprehension that Saddam Hussein, a two-bit Third World dictator we supposedly dispatched over a decade ago, scored the most stunning surprise attack in human history against the United States on 9/11/01, killing 3,000 American civilians and inflicting $200 billion damage on the US economy in one morning -- and there's not a damn thing we can do about it.
Massive fear created by the understanding that there are sleeper agents scattered through the continental United States who could kill millions of Americans and turn our greatest cities into worthless wasteland on receipt of a single e-mail from Saddam Hussein -- and there's not a damn thing we can do about.
Complete tanking of the US economy, as it sinks in to investors that the United States must, inevitably, sooner or later, risk a potentially devastating war with Saddam Hussein, very possibly involving the use of weapons of mass destruction on US soil.
Complete US loss of face in the eyes of the world community, as every tinhorn dictator in the world comes to understand that the US can be blackmailed simply by holding its civilian population hostage using a handful of dust.
Serious undercutting of any future US attempt to persuade Saddam to take exile, once the public understands that he is the author of 9/11 and the greatest mass murderer in American history.
Loss of the "exposure card" in all future negotiations or confrontations with Saddam Hussein. Saddam can only be exposed once.