Would you prefer Obama lobbing missiles and possibly drawing the Russians into a war?
Who's to tell what comprises the Syrian arsenal, who's to tell what Syria has not hidden away, who's to tell any aspect of what two allied, militant anti-Western countries claim they're going to do will do in an effort to mollify the World that the danger of poison gas warfare in the Middle East is now a thing of the past? What are the mechanisms to insure all of this will come to pass?
Unless there's some sort of verification, I can't see how anyone is simply going to take the word of a ruthless dictator whose killed his own citizens and an ardent Cold War warrior itching to show up its now decrepit, age old arch foe?