Down side being that the drive shaft can then "float" on both the trans output shaft and the final drive input shaft when the para-lever is flexing up and down.
As opposed to the drive shaft only sliding at the middle set of splines when the para-lever is flexing.
I think if you look close, the center of yoke should line up with the pivot points of the drive shaft housings, when clipped in place.
If the yokes start "floating" on the shafts then the center line of the joints can move out of alignment with the pivot points. You could then have some strange vibrations. (it's only my theory, I'm no engineer)
I's go ahead and get some clips for the two shafts, then you don't have to worry about it.