Defensive pessimists tend to be very successful people, and their low opinion of the outcome isn't realistic; they use it to motivate themselves to perform better.
Defensive pessimism can be very constructive on the organization level. Financial giant J.P. Morgan Stanley faced the reality that the highly symbolic building they occupied was a prime terrorist target. They responded by implementing escape drills, which they expected everyone to attend and take seriously. This heads-up attitude proved to be lifesaving later on 9/11. In spite of receiving an almost direct hit, they lost only seven employees.