At this point, it's simply not possible for me to believe that the Dems are actually falling for this "I'm so very sorry..." act. The only explanation for why he keeps getting let back in is that they actually /want/ him around to play "bad cop," so that they can say to progressives "we tried, but we just couldn't get that black sheep Joe on board."
I mean, is it possible that, despite public appearances, Holy Joe really is a genuine Democratic team player--it's just that the Dems aren't on our team, here? Think about it: if it wouldn't be for Joe, the Dems wouldn't have an excuse to not pass progressive reforms. If he didn't exist, The Village would have to invent him.