Flexibility is the range of movement that you have in your joints. Some say that this definition should only apply if there is no exterior help to move the joint, but this is not correct. If your joints have a greater degree of movement when aided by an external force, then that degree of movement defines the flexibility.
Hence, if you strengthen you muscles, and these act as the external force, then the flexibility of your joints will allow a greater range of movement than were your muscles unable to exert that degree of force.
To put it simply, if you have a great degree of flexibility in your joints, then the stronger you are, the more you will be able to put that flexibility to use in the range of movement you have at your disposal. It goes without saying that the more flexibility you have in a martial art the better.
However, people do not have the same range of flexibility throughout their bodies. Even more specifically, flexibility in one range or direction of motion at a specific joint does not imply flexibility in another. Hence that fact that you have complete ability to complete perfect front splits does not infer that you have the same...