This is debatable. He fought a very costly war with Iran for years that destabilized the region and cost millions of lives. I think in the long term a functioning democratic Iraq (should it get there) would be a lot more of a thorn in Iran's side than Saddam ever was.2. He acted as a very effective foil against Iran's regional hegemony
While not directly tied to AQ, he had shown plenty of support for Islamic extremists over the years. The war in Iraq also attracted a lot of these types....who were killed by the boatload. AQ actually ended up completely alienating the population of Iraq with its brutality. There was an outright mini-war between Iraqi militias and AQ. Iraq isn't a hotbed of AQ recruitment.3. He kept Islamic extremists like AQ - as opposed to arab nationalists who have never been a strategic threat to the US - from using Iraq as a base of operations and recruitment.
I think this is the main question at hand. Was it worth it? I don't think we'll be able to tell for 10-20 years. People expect results far to fast. We had members of Congress in 2006/2007 outright declaring the War lost....and look what happened after the surge. Americans are too impatient to consider the long term future of Iraq. If Iraq can develop into a stable Democracy in the next few decades, it could potentially lead to other dictatorships in the region loosening their grips. In such a scenario the Iraq war could end up being a flashpoint for change.If we had focused on Al Qaeda and let Saddam continue to do those things he did well, it would have saved us billions of dollars and thousands of American lives.