Difficulty and obscurity are not the same. Technical prose and obscurity are not the same. Obscurity that is for the sake of obscurantism is always wrong if our intention is to communicate, learn and educate, and not mystify and indoctrinate. Some writing needs to be technical in order to communicate accurately and precisely. But most technical language is in fact unnecessary in proportion with the level of expertise required by the subject. Those of us who are for openness and democracy should be skeptical of esoteric language. We should be suspicious that much esoteric language is not in fact the secular equivalent of religious mumble-jumble. Those who write obscurely or esoterically need to assume the burden of proof that there obscure or esoteric style of communication is necessary for the subject. Of course I am only making this case for non-fiction, as will become evident below.
Without knowing why you think "clear speech" is a fetish I am not quite sure how to answer this. The nuns used to tell me to "annunciate, annunciate" and because I was a stutterer in elementary school they used to give me famous speeches to read out loud. Surely, when you speak you speak to be heard by those you are speaking to. Being heard is only one step toward being understood.
And also when you speak or write you speak or write to communicate something, unless you are speaking or writing simply for your own pleasure. If your object is to communicate you don't only want to be "heard" or "read" but understood. Most often clarity is an aid to understanding. Not writing clearly when you intend to be understood is usually an indication of confused thought.
Yes there are topics that are difficult and cannot be expressed in everyday language without sacrificing depths-- mathematics, physics, genetics, etc. There are also genres where the communication of "mystery" or the "density of meaning" is precisely the point. Tolstoy's fictional prose was always clear and deliberately so; the same cannot be said for Joyce. Sometimes in fiction and poetry, ambiguity, ambivalence, obscurity, irony, is precisely the point and at such times "density of meaning" is reflected in the glass-darkly of prose only to communicate more meaning than prose that is straightforward and clear.
This is to say that there are good communicative reasons for "obscurity" that have nothing or little to do with deliberate obscurantism, and there are good reasons having to do with preciseness for technical concepts. But through human history those who claim the need for technical concepts or esoteric language or seek to keep their technical means of thinking and communicating esoteric, most often do so for reasons having to do with elite and class dominance. Those of us who seek a more democratic politics and economics should be deeply suspicious of what seems to be unclarity for the sake of mystification and obscurantism for the sake of esoteric elitism. Much of the technical language and obscure prose used by authors, even those who think of themselves as being on the left, exists to enforce a kind of intellectual exclusivity or to promote academic reputation. I have read nothing written by Judith Butler or Derrida or Zizek or Foucault that could not be written in the prose style of Edmund Wilson or Bertrand Russell, or for that matter the Simone de Beauvoir of "The Second Sex." But writing such things in a clear prose style would often reveal the absurdity or vacuity or mere everyday truth of the thought underneath the writing.
There are times when obscure prose is not for the sake of obscurantism or when language is esoteric but not for the sake of exclusivity. When thoughts are being expressed for the first time and the author is trying to work through his/her thinking in public then even though the writing might not make sense, it may help others to entangle and then untangle a knot of meaning. In such a case obscurity has a creative and germinating aspect and may lead to clarity later on and/or the development of a legitimate subject of expertise and technicality. I often told students that Kant is the only unreadable philosopher worth reading. This I think is an exaggeration. I think I might add Wittgenstein or Augustine, though sentence for sentence neither Wittgenstein nor Augustine are unreadable simply very dense. I think that in the instance of all three of these cases the authors are working close to the limits of thinkable and expressible thought. Much that is good in philosophy (or poetry for that matter) tries to push the edges of thinkable thought in this way and I suppose a certain amount of tolerance in this area is necessary. But we should have our detectors out for pretension and intellectual social climbing.
Jerry Monaco