The problem is that mannequins cannot move in the way that people do. My little mannequin cannot sit. It's hands can't touch its face. It can't cross its arms, tuck its ankles primly, or touch its toes. The little wooden joints simply don't do that.
A mannequin can also bend in ways a human cannot. It's head swivels, it does more-than-spilts, it can stick is arms up... backwards.
Overall, a mannequin is no help in art.
The only purpose I have discovered for it so far is:
But I wouldn't use a mannequin to help draw a unicyclist. I tried, honestly I did. But my drawing-from-imagination turned out much better. I'm much more likely to get my little sister to take a blurry snapshot of me in action than I am to use a mannequin.
I don't understand why mannequins even exist. The imagination is a MUCH better tool-- for me at least.