In the comments on this post, Keshav worries that anti-utopianism may lead to complacency: isn't it a good thing, he wonders, to strive for an ideal, even if one knows one will never achieve it?
Good question. I will try to indicate when I think striving toward an ideal is fine and when it is dangerous. Let us move away from politics, to take the emotional charge off of the topic, and consider basketball (which appears to be coming another major theme of this blog).
A basketball shooter should clearly have the goal of making every single shot, even though he knows he will inevitably sometimes miss. But the player slips over into utopianism if he takes this not as an unachievable ideal by which to orient his practice, but as a realistic goal which implies that he should "never rest" until he achieves it. In the latter case, he may decide to increase his time practicing shooting continually so long as he is not hitting 100% of his shots. That approach will prove destructive, since after a certain amount of practice, he will actually begin doing damage to the muscles involved in shooting.
This distinction carries over into policy quite nicely, I think. It is a reasonable policy goal to strive to minimize the destructive effects of drug addiction in a society. But if we set a goal of a "drug-free America" as something we might realistically achieve, then we get the massively destructive "War on Drugs."
It is easy to provide more examples like the above.