There may be logical reasons why Obama deliberately underplayed this.
For one thing, it defeats the Hannity "uppity" premise.
For another, it activates us -- his base.
For another, since the mainstream media is going to provide false balance ANYWAYS, it allow them to spend more time dissecting lies and material misstatements. And there just aren't many things they can pick at on our side.
I think we'll be content to have that flamboyant, arm-flapping liar win all he wants on "style points". By the time people soak up what he really said, and how he's done another aerobatic set of loop-de-loops around his previous positions, they're going to be happy to come back down to earth with our man.
Consider the alternative outcome: if President Obama had gone nonstop toe-to-toe against the Romney lies. We wouldn't have moved down the field; the entire game would've been labelled a stall. The President would have likely been seen rolling down in the mud with the Republican. But by playing it straight -- with his opponent flailing like that -- the Obama ground game got all it needed out of this first debate.