We might reach the point of diminishing returns, but we have to be responsible. We cannot just leave. We have an obligation to help clean up the mess.
I've done 5 tours in Iraq, and I can tell you that we reached the point of dimisihing returns the day after we pulled Saddam out of his hole. At the point we did that, the regime change phase of the fight was over. At that point the military should have packed up and gone home. What we have been doing in Iraq ever since that day is nation building...and let me tell you, the military is not good at that. We have a metric fuck ton of departments in our country that do know something about nation building, but with the exception of the DOS, they aren't over there. More to the point, the Dept of Defense is prolly the last damn department you want doing nation building...we haven't done it well since the Marshall Plan, and some would argue that it was screwed up as well.
The plain and simple truth is that when we leave Iraq, the country will disolve into civl war. It's gonna' happen, and it doesn't matter if we leave tomorrow or 10 years from now. Sometimes you have to let the course of human events take place. Right now we have a finger in the dike of those events, and we're getting too many good people, my friends, killed in the process.
BTW, right now, I'm deployed to Afghanistan doing....you guessed it, Nation building.