I thought Biden was sort of a Bush 41. A Veep in the background who didn't really have any defined constituency. Bush 41 only won because the Dems imploded.
Biden was fairly invisible and I'm not sure what he did or accomplished in 8 years as VP. [Al Gore at least was an active Veep and had ideas -- but somehow got creamed in the media day after day). At least Biden would have spent time defending the O record, might have had O campaign a little, and probably would have given a defense of O-care. But I think Biden would have risked getting beat by Bernie, and wouldn't have generated much enthusiasm.