U-joint on steering shaft: float or bolt down?

I was wondering. Many on the forum love their U-joint (I have the Borgeson one), myself included. Pot coupler believers say U joints don't account for flex in the steering system as the pot couplers were designed to do.

So, question: why not allow the splines on the steering box end of the U joint to float, rather than tightening down the set screw? Just like a drive shaft yolk slides on the transmission output shaft to account for flex/angle change in the drivetrain, why not float the U-joint on the steering box shaft splines?

Brian
Well my dad's 64 valiant coupler slipped off and he ended up in the ditch, upgraded to a ujoint tightened down and much safer.