We don't think flexibility is only a matter of stretching one muscle or joint range of motion. Although joint range of motion is important, it can be overdone, leaving the joint hypermobile or even damaged.
Separate muscles work together or against the other in a positive functional manner. However, when muscles get out of balance, they can cause a joint to be hypermobile or hypomobile, which can result in poor performance or injury.
Over-stretched, loose muscles can become weak. Tightened, hypertense muscles can cause bones at a joint to pull out of alignment, which can cause damage to the joint surface, or decrease athletic performance. Parallel muscles can experience trouble if the designed weaker muscle is abnormally weaker.