I would disagree: I think that there's no political motion without political independence. If the left is going to support Obama, its going to have to tone-down or drop any demands that Obama opposes, since simply bringing up those issues would expose the gap between Obama and mass sentiment. If there were a high-profile rally for single-payer health care in the last week of October (as an arbitrary example) that would be a real embarrassment to Obama, so no one is calling for one. Concretely, political motion is dampened so as to not inconvenience the democrats.
The experience of the anti-war movement in 2004 really confirms this. How could the anti-war movement have both endorsed Kerry and continued to oppose the war? Verbally it managed to, but concretely, in terms of its actual actions, it totally dissolved itself for the convenience of the Kerry campaign.
There is certainly room for flexibility in approach. Arguing that the anti-war movement stay "independent" is a lot more productive than arguing that it endorse some 3rd party and denounce Obama, which would only isolate the movement and split it internally. Arguing against voting for Obama may or may not get you anywhere, depending on your audience, but arguing that building the movements is more productive than building the Obama campaign is certainly a useful argument, and its one thats likely to convince people. These tactical choices are what will determine if "you['re just] waging the finger from the sidelines" or doing something useful.
The real "paralysis," IMO, comes from supporting someone who has interests hostile to your own.
Mike