First we need to define stretching. Yoga, Tai Chi, etc. are not considered stretching. These activities which involve specific postures / slow well-defined movements that are often accompanied with repetitive breathing and may have a spiritual emphasis do not fall into the classification of classical stretching commonly done before or after vigorous exercise.
The first question is do we need to warm up / stretch. Below the age of ~ thirty, the answer is simply no. Our nervous system, muscles and vascular system is capable of immediate activity. It is part of the survival mechanism of life. On the other hand, after the age of ~ thirty, it is simply considerate to our our vascular system and heart to kind of prime the pump and slowly raise the pulse and blood pressure. It is just a courteous thing to do as we age and our blood vessels lose some of their elasticity.
So what's the answer regarding stretching?
My colleague Dr. Phil Maffetone says it best in his book,
The Big Book of Endurance Training and Racing, "...My recommendation has always been to include an active, aerobic warm up as part of each workout or race. This can be accomplished through slow running, biking, swimming or any aerobic activity that last at least 12-15 minutes. In addition to improving oxygen utilization, lung capacity, and fat burning, it increases flexibility in a safe way. Stretching cannot do the same."
There is one exception, dancers. Here there is a need for balancing of both negative and positive physiological factors. Warming up movements can loosen the ligaments and muscle/tendons to maximum flexibility but at the same time there is some loss of top-end function/strength and an increase in the potential for injury immediately following the stretching activities.
Recent scientific studies are confirming this fact that the potential for mechanical instability (joint sprain) and muscle /soft tissue strain is dramatically increased with the stetching movements that we all previously learned.