The prop-shaft assembly has to deliver a shed-load of torque from the gearbox to the back wheel. A little wear & tear could be reasonably expected over years of use. When the time comes to refurbish this essential transmitter of power, the more parts that can be renewed the better and more durable the refurbed assembly will prove to be - even a worn 2 euro washer could be vital!
Other prop-shaft replacement parts you may wish to consider:
Renewing even a worn or damaged washer will add to long term reliability.
Add as many items as you wish to the same shopping cart, new fasteners to ease reassembly for example, and still only pay the same fixed price shipping cost.