Really, I'm surprised this hasn't been mentioned. See, McCain needs to win Ohio, so adding Taft is a great way to lock up the state. After all, adding a VP from a state always guarantees the state will vote for that ticket, period. Just how John Kerry won NC in 2004 as well.
Sarcasm aside, a quick look at the Atlas shows a real swing in, for example, NC in 2004 and WY in 2000. Picking someone popular in a state that's on the tipping point could be a good idea. Picking someone unpopular or trying to get that effect in a large state is obviously not so bright.
Wait, did I just feed the troll?