First, even if the nationalists and Baathists turn on us after we jointly defeat al Qaeda, the Sunni Arabs will be without their most vicious and determines allies. That hardly seems like the way to win a war.
Second, I wrote that we are filing away the identities of our new allies, so a return to enemy status will make their lives short and interesting. This article notes this program:
U.S. commanders say they collect the names, photographs, fingerprints and retinal scans of all the former insurgents they work with and require that they agree to a set of rules. They hope to eventually integrate many of these auxiliary forces into the understaffed Iraqi police force.
If these people return to fighting us, we have their information in databases and any time they are stopped or have any interaction with authorities, they risk getting caught. Giving up this kind of information is a pretty solid indication that they will stick with our side.
Finally, it amazes me that it seems like the same people who say we can't defeat the enemy on the battlefield are the ones saying we can't accept their defection to our side. I guess they get around that problem by asserting we must lose.
All along in this counter-insurgency, I've written that we have to pull hostiles to neutral status and neutrals to friendly status. We are pulling Sunni Arabs our way along the enemy-neutral-friend continuum. This is called "winning." Try to get used to it.