Plenty of people have made the case against Barack Obama picking Jim Webb as his running mate on the basis that it would be bad for Obama--it'd hurt him with feminist Clinton supporters more than it would help him with the white working class, Webb isn't much of a campaigner, etc., etc.
Jim Fallows, who's known Webb for nearly thirty years, makes the less intuitive case against the pick on the grounds that it would be bad for Webb:
Jim Webb has arranged his life so as to maximize his intellectual and personal independence, and minimize the things he "has" to do and the bosses he must answer to. Novelist, essayist, journalist, movie-maker -- through the two decades before his Senate race he's been his own boss as much as possible, and has clearly relished saying exactly what he believes. The federal government office that most nicely matches his previous life is the one he now holds: as a U.S. Senator. Especially a Senator of the model Webb has described as his ideal: Daniel Patrick Moynihan....
The federal government office that least matches Webb's lifetime path is the vice presidency. Some wonderful people have held the job, plus some terrible ones. The ones who are happiest are those who can bide their time, bite their tongue, fly to foreign-dignitary funerals, and stick absolutely to the company line.
--Christopher Orr