Budget, duhh. KB's cost more. Plus you have to have file fit rings. They cost more. Plus you have to have the sheen shop file them. Cost more. It is easy to do I know, but I am assuming a newbie here who might not quite have skills yet to do such. Lastly, there is no chance for any type of funky failure in the event "someone" doesn't get the ring gaps right.
JMO that the KB pistons are not needed here.