"Whoever is not with us is against us" came from a speech Lenin delivered in 1920. According to CNN, President Bush actually said, "You're either with us or against us in the fight against terror." In an e-mail exchange with someone on the other end of the political spectrum from Mr. Bush, discussing the attitudes of non-conservative Christians, that person said "personally I believe they are complicit if they don't take a stand."
If you draw a circle and define those outside it as the enemy, you can create a siege mentality and unify your power base. But the price for the tactic is high: it allows no room for neutrality and risks alienating those not yet aware of the conflict who have not even taken a stand.
There is also sort of arrogance in this approach. Note the underlying assumption that the speaker's issue, whatever it might be, is so important that the entire world must know and care about it to the same degree that the speaker does.
What's the alternative? How about "whoever is not against us is for us"? (Mark 9:40) Or at least recognizing that there might be people in the world who are busy enough with with their own problems that they're not ready to adopt a new one.