"[Relation Pronoun] + [be V] + [adj] => [adj]" means that certain "be" verb expressions can be left out without changing the meaning, which is definitely true. "be" is like the most common, "default" situation of all verbs, so in certain constructs they can be omitted without any loss of information. The most common example is the relative clauses as you listed.
For example, in these relative pronoun sentences, omitting "who + be" leaves the sentence equally clear (if not more).
1. There were many people (who were) stricken with this disease.
2. His wife, (who is) a very nice person, has helped me.
The other major case is with participial clauses.
1. (Having been) promoted twice, he is now a mid-level manager.
2. (Being) an efficient person, he did not wait until the instruction came.
To me, these sentences sound even better without the "be" participle.