I am a worker from/in the UK, but if TRM.40780 says, "Maternal RhIG candidacy assessment must include the identification of weak-D phenotype newborns", that is exactly what it means. It doesn't say "should" instead of "must", and it doesn't say, "until you give up because you are bored, because you have never found one"!
Yes, such types are rare, but they do happen, and they can cause the mother to produce an anti-D (of sorts). These antibodies are not usually particularly clinically significant in terms of further pregnancies - but the word "usually" is the important one in that sentence.
The only thing I would say is, WHEN you do detect a newborn who tests as weak-D positive, don't forget to test the mother too; she may also express the same weak-D type (but, depending upon your laboratory's policy, may not have been tested as expressing this type during the pregnancy), and, if she does, she doesn't need the anti-D immunoglobulin (which, remember, is a human-derived blood product, which may contain a novel blood-borne virus type about which we know nothing - YET).