not only that.. but dont most aluminum/steel or framed stencils usually have some sort of 'machine' to position the frame over the board(s)?
maybe the rigidity of the stencil means you dont have to have other blank pcb's around the target board to make up for the mylar/transparency 'flop'..
@$20.00 for aluminum/laser cut.. they are finally making a run at regular mylar/kapton cut stencils price wise!