One of the drawbacks to using a bolt is the bolt stops very short of bottoming out in the crank, it has to do this to ensure you get a good clamp load on the balancer. So the bolt is not fully engaging all of the crank threads and in turn is not as stable as a stud fully threaded into the crank. Think of it like this, if you were building a new motor would you want to use bolts or studs for the heads?
I look at it like its cheap insurance for preventing spinning a balancer and destroying a crank/motor but to each there own.
