In the past at least, rcv (and many other shaft builders) would cut splines into a blank and not taper the shafts down to the minor diameter of the splines.
It works fine for most people, and is still stronger than the joint/yokes in most cases,
But on a splined shaft, there is significant toughness/resilience to be gained by profiling the shaft.
Essentially making the shaft into a big torsion bar rather than just a bar with splines