Well, Jarmo, there's a different way of looking at this. I think the use of mannequins for art can help tell us about our culture when we, not the stores use them satirically.
"A mannequin named Bonnie, standing with one hand rammed through the glass of Bonwit Teller window, answers questions from people on the street; a male mannequin in a window at Barneys towers menacingly above a black leopard and a reclining female mannequin; mannequins dressed as call girls catch a last few winks in an early-morning bedroom scene staged at Henri Bendel--show-window "theatre" can be provocative, shocking, wicked, delightful, amusing." is from a synopsis on Sarah Schneider's book,
"Vital Mummies: Performance Art and the Store-window Mannequin" available for $1 to $7
here!
Man Ray and Helmut Newton used mannequins in their photography this way. This mildly
NSFW link is very informative and helpful.
What's helpful for us is that you do not have to pay the model, secure a chaperone, provide food or wait for her to turn up! She also doesn't need a 15 minutes break every hour. Moreover, you can actually move her leg or arm or rotate her any way you please without stepping over boundaries of etiquette or decency! Manequins are also good just to get experience with your lighting and composition and use as a sketching medium. If the image is perfect and you need a real head, or a second live person just do it! Hmm, that's an idea I might consider for myself! Why not if the picture works for you perfectly!
Asher