The argument to the contrary is no different in principle from the arguments against taking action against the Taliban prior to 9/11, with the only difference being that Iraq under Saddam was far more dangerous than the Taliban on its worst day. We should have learned our lesson the first time around.
As for WMD's - Syria has a more advanced program, Iran has a more advanced program. I would wager Pakistan has a more advanced program. And North Korea says they have nukes but might even sell them and tells us to go fry ice! Iraq was no real or immediate threat to us. Sadaam would have loved to be our best friend again. Iraq's WMD's was the excuse to establish a beach head in the war against terrorism.