According to McAtee and Charland flexibility is very important for everyone including athletes. Especially is imporant for the hip and legs. They go on to explain that when a muscle is shortened and not stretched out it is unable to be used to its power potential and limits the range of motion that is achievable.
Think about a runners stride. When the runner has tight hamstrings the quadriceps will work harder because they are working against the hamstrings to compensate. This causes both the quadriceps and hamstrings to fatigue and sets the stage for a poor performance or makes the runner prone to injury.
Also tight hamstrings are a huge contributor to lower back pain, knee pain and leg length differences. If the hamstrings are tight and don't fire properly, neither do the gluteus maximus and this causes the erector spinae to contract firs, followed by the glutes and then the hamstrings and this puts massive stresses on the lumbar spine causing back pain.
SO STRETCH! ESPECIALLY THOSE HAMSTRINGS!