I didn't think Modine's character was superfluous. He represents Gotham City.
At the party he serves to communicate how soft and complacent Gotham has become to the audience. He's there to show that the GCPD is putting on a new, slick face and phasing out the old vets. He helps define Gordon as a shabby relic from a meaner time in the city's history.
During the chase after the stock exchange job, he's there to explain to the audience why the cops are letting Bane and his men get away to chase Batman. It's useful at that point to put a face on the behavior so the audience doesn't attribute that misguidance to all of the policemen. Remember, we're going to cast most of them as righteous footsoldiers later on.
Once Gotham is under Bane's thumb, they use him to tell the audience why people are keeping their heads down, and again serves as a counterpoint to Gordon. They use him to play Gordon up as he risks his life showing his face above ground to urge more people to resist. Then, once the troops are mustered, he gets to be redeemed.