It's because the screw is designed to pull on the metal bush in the middle, which is supposed to expand like a rawl plug under the tension, eliminating the play in that joint.
Can you use a nut and bolt? Course you can. You may get more play than usual, but then when you consider the aftermarket parts available at the moment and the play in the rubbers/frame etc, i'd say it wont really make a noticeable difference at all.
Sent from my iPhone using Tapatalk