I think what's objectionable about that rhetorical tactic, to me, is that it denigrates the exercise of conscience; in fact, it labels reflection and conscience at best enablers of evil, and at worst intrinsically evil. It celebrates groupthink and knee jerk tribalism over the exercise of the intellect.

And it's a gratuitous insult. When Hitler's true colors became apparent, nobody needed to be bullied into opposing the monster, and that "for us or against us" formulation was, in fact, a deadly insult. Change the name to any monster, and it's the same. Berating people with that insulting line can never lead to constructive results.

It's a special case of the broader question of whether people can be moral without an external system of rules being imposed upon them, that system usually being religion. I believe that you can arrive at a stronger morality by thinking through the issues, and refusing to be lazy and just go along with the conventional wisdom. The moral understanding you have to work for is the one you'll stand by when the chips are down.

Osama bin Laden was that kind of monster. But I arrived at the understanding completely independently, and after rejecting Bush's vile demagoguery. And I continue to hold that position even now that Bush isn't around to constantly remind me.