Certainly it was a political tactic, to run up the number of U.S. guns recovered in Mexico to add statistical credibility to Hillary Clinton's claim that U.S. gun dealers and our "lax gun laws" are to blame for violence in Mexico. Their claim that it was to build a case against the cartel bosses is just ridiculous, there is no way that could work. In the first place the big guys don't have to settle for semi auto AK-47's, they import and export drugs by the tons, does anyone believe they have to get their weapons one by one from a crooked gun dealer in Phoenix? No way, those guns were only going to the street level thugs who didn't have the connections to get supplied by the cartels.
The only weapons they could trace were those recovered by Mexican police and they could only trace those to the straw buyer who made the purchase. But that's all they ever wanted, just to run up the statistics. It was working as planned up until the death of Brian Terry, then the whistle blowers came forward and it's like someone pulled aside the curtain and exposed the great wizard as a little guy who was turning the wheels and pulling the levers.
But the curtain hasn't yet fully opened so that we can see the guy behind the wizard.