Scheiber makes a good point here-

to the extent Biden has sometimes gotten tangled up on the subject of race (see here, here, and here), I think it actually helps Obama. There are a lot of voters out there still getting their heads around the idea of an African American nominee. Among other things, they may not yet have the vocabulary to discuss Obama's candidacy without expressing some of their thoughts inartfully, and they don't want to get pilloried if they trip up. The Biden pick tells them, in effect, don't worry about it, this is tricky for all of us. It reassures them that an Obama presidency doesn't mean every word they speak will be parsed by the sensitivity police. And, perhaps most importantly, that Obama himself isn't fazed by an awkwardly-worded sentiment. This strikes me as pretty critical.