I would think that if it is a sit spin, you're risking getting the skates caught in the skirt, plus, it would drag on the ice in the spin. I do know that my daughter's freeskate coach requires her to change out of her dance dress before attempting any freeskate moves for safety (dance dresses being knee length and below). Plus, if the skirts are heavy, then, the weight could throw a skaters off balance.
Generally, I see freestyle dresses "cut off" at mid-thigh.