We have an epidemic of bad posture
How often do you find that a person with a low body-fat ratio with negative effects caused by bad posture?  The people who I've seen with obvious problems caused by poor posture, like a hump or sore back or joints, are usually moderately to highly overweight.  Kind of like how women with large breasts can have problems with soreness or neck humps.  I suspect that it's not so much about good posture as it is about having a variety of postures throughout the day and not having bodyweight stressing one's joints.  Granted, I have no scientific evidence for this.  It's just from my own observations(and my own experience of being a former fat guy).