The hamstrings serve as both the flexor of the knee, and the extensor of the hip. As a result, the hamstring is critical in any activity involving walking, running, or sprinting. Muscle imbalances can involve the quads being far stronger than the hamstrings, or the right leg being stronger than the left. Further, there can be bio-mechanical problems with running style, or jumping that create a higher risk of injury. As probably the longest and least stretched muscle of the body, poor bio-mechanics that are a result of another injury, say to the ankle, can create a wildly pronated gait that enhances risks.
Excess pronation can place stress on the knees, lower back, and hips.
Since the hamstrings traverse the hip and knee joints, this exposes the these muscles to absorbing tremendous pressure. A overly exercised quad with a hamstring that is ignored will result in an inability to push off, sprint, or even run at a slow pace. If you are not going to strengthen the opposing muscles, don't lift at all. Not only is an injury to the hamstring likely to result in significant down time, but improper rehab can create increased risk of reoccurrence. Generally, minor strains can take weeks to return to action, while more serious injuries can take months.