Actually, the wiper arm comes new with a smooth walls in the hole. The "splines" are where the relative soft arm material has mated with the serrations on the steel spindle. As long as the arm is tight when the nut is torqued, everything is fine. Unfortunately, sometimes the nut comes loose and the spindle reams out the hole in the wiper arm. This will result in the arm being loose after the nut is tightened. If this is the case, replace the arm.