The purpose of the slots in the female splines, the taper on the OD of the female portion, and the nut (which has an internal matching taper), is to LOCK the two splined sections together. It should be obvious that the shaft is splined to provide, 1. an initial length adjustment to allow for manufacturing variances, and, 2. a means of reducing the length of the shaft for service work.

The flex discs incorporate enough play to absorb longitudinal movement of the engine/transmission, and the final drive center section. This method is in contrast to the deliberate slip joint method employed with live axles.
The MB splined joint is NOT a slip joint.