I'm not sure I agree with the specific claims Jonathan Cohn is making here about Sebelius and Webb, but he does
make the correct meta-level point which is that it's a huge mistake for activists and so forth to think about the veepstakes primarily as a question of short-term political tactics. Becoming Vice President of the United States dramatically increases the odds that you'll become President down the road, so it's actually really important -- more important than is generally recognized -- that the person who's picked be someone who's substantively good.
This article available online at:
http://www.theatlantic.com/politics/archive/2008/06/the-high-stakes-of-veepstakes/45420/