In a partition, the rebels will continue to assemble a government and maintain oil sales, while Qaddafi can't. Eventually they'll take care of him. Given that we've created a no-fly/no-armor/no-artillery zone, if the rebels can't fight off infantry at this point, then they don't have the support and drive to maintain a revolution.
As for Josh Marshall saying no genocide=no humanitarian reason to intervene, I think the rebuttal argument makes itself.
It's definitely a roll of the dice, but doing nothing isn't such a great play either. And while eastern Libya has been a hotbed of Al Qaeda recruitment in the past, a chance at a more democratic society could help there, and meanwhile we're destroying every weapon in Libya that can shoot down aircraft.