I guess they wear them that way to pull them down faster.
Dancers (in training) wear PH or Tights not because it looks good, they wear them to keep the muscles warm after warm-up. But then they will get them out at one time. For a similar reason they wear tights with a hole in the sole. When dancing in tights you may slip, but when dancing barefoot you can not slip. So they pull up the tights and dance barefoot while still wearing the tights (to keep the legs warm!).
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