For BPE the situation relates to production costs. Many engines use ball stud rockers that function very well. Are shafts more stabile? Yes. But for a street focused engine the stud mount are good for the cost. Also remember one of Johnny Mac's requirements was pushrod oiling. So that precludes the arguements. Shaft rocker assemblies are available for Ford and Chev applications that came originally with studs. For racers you can spend the required $.