We know bad things can happen no matter what we do, but bad things are already happening every day. If we stay, the brutal and senseless killings continue, and we lose more soldiers, contractors (Some people do care about them), and money. If we leave, the killings may continue, there is no guarantee they will stop, but we will lose less troops and spend less. Iraq has already said it's ok if we leave (they were polite enough to no add "you've done enough damage"), so let's graciously accept their offer.
There is a chance that some of the violence will subside if we leave, as much of it is directed at the U.S. presence there. We don't know what will happen, but we do know what will happen if we stay: more soldiers will die, hundreds of billions of dollars will be wasted, and the U.S. standing in the rest of the world will continue to decline.
Leaving is not a great solution, it is not a good solution, but it is better than staying. Ethanol is not a great solution for powering cars, but it is better than gasoline. Kerry was not a great choice for president in 2004, but he would have been way better than bush. In all of these cases, the defenders of the status quo make their case by pointing out the dangers of making changes, while ignoring the absolute certainity of disaster while staying the course. When did this country become the fearful nation?