Trained, well-equipped military personnel have a very high chance of surviving a chemical or bio attack. Those weapons are most effective nowadays against civilian populations, such as the Tokyo sarin attacks or the gassing of the Kurds. In other words, they're nasty terror weapons, hence the need to ensure that Saddam no longer had them, since Saddam was not willing to prove that himself.
These kinds of weapons don't have to produced by a rogue nation with a "weapons program." For civilians in the U.S., Iraq was never the biggest threat from such an attack.