Barack Obama is supposed to be the first post-baby boomer candidate. So who does he choose to carry on his work if he dies or is incapacitated? Someone who was first elected to Congress during the Nixon presidency! While Biden is younger than McCain, both are pre-boomer.
Biden is also completely entrenched in the partisan Washington culture that Obama insists he will change. Biden's big draw is supposed to be his foreign policy experience. Yes, he has been on the Foreign Relations Committee but he also has a long history of being wrong on international affairs. Most recently he tried to divide Iraq into three pieces. This was one thing that the Iraqis united on - the thought it was a stupid idea.
Granted Biden is more experienced than Obama but that isn't a difficult accomplisment. The entire Senate is more experienced as are all state governors and former governors.