At the Atlantic Special Ideas Report, Conor Clarke makes a case against polling, for, among other reasons, polls' ability to influence mass opinion by reflecting it, accurately or inaccurately, and to effect a herd mentality:

[O]f perhaps greatest concern: the outcome of one poll can affect future polls and behavior. As behavioral scientists and economists are fond of pointing out--in books like Nudge and Predictably Irrational--popular behavior can snowball. Public-health campaigns emphasizing how few teenagers smoke are more effective in deterring teen smoking than those that emphasize lung cancer or bad breath. Likewise, the perception that a candidate or political position is popular today will make the candidate or position more popular in the future. As Cass Sunstein and Richard Thaler put it in Nudge, "Nothing is worse than a perception that voters are leaving a candidate in droves." Voters should be free to switch allegiances whenever they want, but they should do so for substantive reasons, not because they're following the flock.

Most everyone acknowledges the problem with polls when it comes to Election Day: exit polls are frowned upon and in some cases banned, because early ones have been shown to influence the behavior of people who haven't yet made their way to the voting booths. If we can see that it's a problem on Election Day, shouldn't we acknowledge that it's a problem the rest of the year as well?

Polls illustrate, in other words, the power of perception in politics. Fatigue sets in when one's favorite candidate is down. If he's down ten points, why even take the trouble of voting? In that sense, polls corrupt the experiment of an election by suggesting results beforehand; for all the statistical science that goes into them, they're fundamentally anti-scientific. And perhaps more significantly, Conor suggests, people can change their opinions to side with the front-runner.

That's why political groups commission polls with questions that shade the issue in their favor: they want to look like they're ahead. It's also why we have to be careful in reading the results of polling, so we know just what the numbers say.