As far as I know, every language has forms of expression specific of the written/formal language. Actually, I would say that in spanish the spoken language is slightly closer to the written language than in many other european languages. As for the examples you mention:
-It is not true that the perfect future is not used. As a native speaker I can tell you that it is widely used. Example: "Para cuando llegues ya habremos terminado de cenar" it is a perfectly normal sentence , which doesn't sound far-fetched or anything like that and which makes a crucial use of the perfect future in the first person (plural) ("habremos terminado" = "we will have finished", literally)
-The future subjuctive: It is true that it is not very common, BUT, as you mention, it is common to find it in written language and also in some sayings and expressions in the spoken language. For example, "donde fueres haz lo que vieres"(equivalent of the english saying "when in Rome do as the romans") or similar constructions. I would say the future subjuctive is common enough and useful enough so that calling it obsolete makes no sense.
An interesting example, however, is how the imperfect and perfect simple subjuctive past tenses of latin merged into one single tense in spanish, thus making the two forms "fueras" and "fueses" equivalent. In this case spanish DID lose something, namely, the perfective aspect of the simple subjuctive past, whose role can be played by either its imperfective counterpart or the subjuctive pluscuamperfect ("hubiese sido"), both very common in conditional constructions.