Insider trading is a classic example of the instrumental value of information and it reminds us of how markets are socially constructed (not in the sense that they are figments of the collective imagination but in the sense that they generally need the support of some social norms, rules, and laws in order to function well.)
The Times just reported (in "S.E.C. Accuses Mark Cuban of Insider Trading") that Mark Cuban, owner of the Dallas Mavericks, has been accused of selling all of his shares in a company a few hours after receiving confidential inside information suggesting the stock price would drop. At first glance it would appear that the "bottom line" is the monetary value of the information (that Cuban had but others did not), but consider how a source explained things, as quoted by the article's writer: "It is fundamentally unfair for someone to use access to nonpublic information to improperly gain an edge on the market." The "technical" reason for prohibiting insider trading is that markets can fail if people think others have private information, but this comment suggests that there's a social norm at work too: it's just not informationally fair.