What Could Possibly Go Wrong? Investigation: US ATF Secretly Arming Mexican Drug Cartels (Mar 3, 2011 - CBS)

Well, empirically, let's list some hypothesized outcomes of "letting guns walk":

1. The guns end up in the hands of cartels, the ATF and DoJ build a case, the cartels go down.  After 40 years, major strides are finally made in the War on Drugs.

2.  Massive numbers of assault rifles are smuggled across the border.  Violence and death tolls greatly increase in Mexico. There is no case built up against the cartels. Guns continue to walk. Obama praises the reduction of gun violence in Mexico.  The War on Drugs continues.

3.  If the ideal outcome is not the actual outcome, at least the ATF have given themselves, and the cartels, the impetus to maintain an endless, fruitless war.