Vice-Presidents are usually picked on the basis of what they can do before election day: make up for a candidate's percieved weaknesses (a specious argument I won't get into), carry their home state (It's happened before, but hard to prove in a lab setting), and adding balance to the ticket (same as the first reason).
I personally think that a VP should accentuate the positive instead of trying to eliminate the negative on a ticket. After all, if something goes wrong, this is the person who will lead us for the rest of the next four years. Which brings me to my main point....
After the election, the winning ticket's VP serves as a form of assassination insurance for the president: basically, the idea of this person becoming president should ward off potential assassins motivated by politics. Since ideologically motivated political assassins come from the opposite end of the political spectrum, it's a good idea to have a VP who is slightly more extreme than you.
Dick Cheney is a good example of this. Andrew Johnson, Lincoln's VP, is a good example of the opposite: Would John Wilkes Booth have assassinated Lincoln if someone like Thaddeus Stevens was VP?