The arguments against containment are many: (1) "Saddam as a serial aggressor bent on dominating the Persian Gulf;" (2) "Saddam has used WMD against his own people (the Kurds) and against Iran and that therefore he is likely to use them against the United States;" (3) containment "is unlikely to stop Saddam from getting nuclear weapons;" (3a) "Saddam would give nuclear weapons secretly to al Qaeda or some other terrorist group;" (4) a combination of all of these. Since these are true, a war to disarm him is necessary.
A case for containment is made here by John Mearsheimer and Stephen Walt. Another must read.