When I tell people that I'm a yoga instructor, sometimes the response I get is, "Oh, wow! Yoga! I could never do that! I'm so inflexible."
Usually my response is, "That's OK! Yoga is where we learn to be flexible ... among other things." And then sometimes I go on some rambling monologue about the myriad of benefits of yoga.
Anyway, I'm here to tell you that being naturally flexible is not all it's cracked up to be! Sometimes I wish I were strong and stiff instead of flexible.
For those of us blessed with natural flexibility in the joints, it can be easy to replicate the look of a posture without doing any work. For example, take Dandasana/Staff Pose. If you just look at the shape, you might find it fairly easy to sit with your legs in front of you, in the shape of an L. But this posture can become a lot of work! Engage your feet, pulling the arches toward you, pull your kneecaps toward you, tone your belly and keep lifting your chest with your own strength. Oh yeah, and keep breathing! This is where the yoga is happening, finding all the nuances of the posture. Also, keeping your muscles working is essential to protecting your joints.
I would go so far as to say that yoga is even more beneficial for inflexible people! My hypermobility has caused me injuries ever since I was young. When I was in high school, I suffered for a long time with a knee injury, and was constantly twisting my ankles if I stumbled even a little bit. Western approaches like orthotic inserts for my shoes and physical therapy did solve some of my problems. However, I have recently noticed that in spite of being a yoga teacher, I don't always move mindfully when I'm off the mat! I have thrown out my SI joint probably by getting out of bed some sloppy way, and I know that I often sit into a chair in a way that is not so awesome for my knees. I can't always count on my joints to maintain this natural (or unnatural!) flexibility. Muscles must be worked long and strong both to allow joints to work the way they're supposed to, at the maximum range of motion and with the maximum amount of support. This is how we train our bodies to serve us safely for many years. Some us (waves hand) have to work harder on the strong, and some of us have to work harder at the long.