note the word 'required' vs. 'recommended' (...I imagine when there's a failure...the word 'required' will come up with an exclamation point)

...you may wish to consider and note your appetite for risk ..are you risk adverse on this matter?

..if so, you may wish to proceed with a maintence service/review in this area...ie having a tech look at the plastic rails and/or show you them & have the chain measured for stretch (compare to range)...that info will offer better data to decide on proceding with these items

hope this helps
-fad