No one can bring down a party like a pessimist.

Their focus on the worst possible outcomes has given these individuals labels like Debbie Downer, Negative Neil and Dr. No. While it’s easy to overlook their cynicism, and perhaps even make fun of their gloom and doom attitude, those who see the glass half empty may be just what your company needs.

While we’d all love to think every project will be successful, every opportunity is a golden egg, every risk taken will result in a reward; the reality is even the most successful organizations will fail sometimes. The value pessimists bring is indeed their ability to expect the worst possible outcomes and to see the potentially dangerous obstacles in every opportunity.

A pessimistic attitude can be a difficult one to accommodate, but as Megan McArdle points out in Businessweek, an in-house critic can be a company’s most valuable, and often most overlooked, asset.

The dangers of ignoring a cynic are no clearer than the example of the NASA space shuttle Challenger in 1986. Engineers worried that the cold weather forecast would cause problems with the rubber O-rings that sealed the joints in the solid-fuel booster rockets and recommended the shuttle’s launch be delayed. Excitement over the scheduled launch overshadowed the decision-makers who argued the engineers were being too cautious (read: overly pessimistic). NASA launched the shuttle and the O-rings failed, killing all seven astronauts on board and nearly ending the Space Shuttle program.

The Challenger example demonstrates what McArdle calls “groupidity”–a situation in which a collective become more willing to take risks because everyone in the room appears to be on board with the idea, whereas individually they may think the decision is stupid. As social animals, most of us like to feel part of a group. Since dissenting from the prevailing attitude doesn’t come naturally to us, pessimists are often labelled as anti-social, sometimes ostracized from the collective, making it easy for their warnings to be dismissed.

Constant negativity, though, can at times be more of a problem than a solution. You don’t want the Negative Nelly to kill every project, but inviting her to the board table can provide some valuable insight into the potential dangers of moving forward with an idea. “Think of the Voice of Doom as your defensive coordinator, identifying all the holes you need to plug, and backup plans you need to have in place before you launch,” writes McArdle. As the Challenger example shows, they may sometimes be right.

Does your company have a voice of doom? Invite them to your next meeting.