My husband, who played college soccer and attempted to go pro after graduation, has been arguing with me about this very subject for years. I repeatedly chastise him for not stretching before running/exercise, and he claims it doesn't do anything.
It turns out, he is right. I have read in several different sports/fitness/outdoor magazines that the idea of stretching before working out or activity of any kind, while it does increase flexibility, it likely does not decrease the risk of injury. Soccer players are most likely to sustain injuries dealing with the legs and feet. Torn ACL's and sprained ankles are most common, neither of which could have been prevented by preliminary stretching.
Medical experts agree that the best prevention of sports related injuries are better warm ups (actually fully heating the body before engaging in vigorous activity), core strengthening (both back and ab muscles), general strength/weight training, and balance exercises.
The best time for stretching is actually after physical activity, as it is helpful in cooling muscles down slowly and aids in the prevention of muscle cramps and general soreness.