I've been thinking of contrastive clauses recently, and as it happens an interesting example of the importance of contrastive phrases came up recently. You may be aware of the recent uproar about Chick-fil-A and gay marriage. According to certain news reports, "'Guilty as charged,', Cathy said when asked about his company's support of the traditional family unit as opposed to gay marriage." The problem is that if you actually compare reports, you realize that Cathy wasn't asked about "his company's support of the traditional family unit as opposed to gay marriage" and doesn't seem to have said anything about gay marriage; rather, he seems to have been asked a vague question about his company's explicit support of the traditional family, and he responded with the vague answer that Chick-fil-A was guilty as charged, and were very supportive of the traditional family. And as Terry Mattingly points out, this doesn't tell us anything about gay marriage. Whatever Cathy himself may have meant, 'supporting the traditional family' and 'supporting the traditional family as opposed to gay marriage' are two distinct things; they do not have the same logical implications and they really do need to be distinguished.
In any case, I find contrastive clauses interesting, since I find reduplication interesting and contrastive phrases are clearly kinds of reduplicative phrases. Reduplicative phrases are interesting in themselves because they can often have significant logical effects. Ignoring a reduplication can result in ignoratio elenchi, for instance, which shows that reduplication affects what's being talked about. Likewise, if for any S, R, T, and P, and for a Q that is inconsistent with P, I say, "S qua R is P" and "S qua T is Q," it's entirely possible that the two are consistent, even though S is P and S is Q, without the reduplication, are inconsistent. For instance, "Mary as mayor has the authority to close the street" is not inconsistent with "Mary as concerned parent does not have the authority to close the street". Another example of somewhat different construction: "This theory T is capable of adequately explaining the U.S. flag as a physical object" is consistent with "This theory T is not capable of adequately explaining the U.S. flag as a social symbol."
Contrastive phrases work reduplicatively in this way. If I say, "This theory T explains physical objects rather than social symbols," the "rather than social symbols" adds something of considerable importance beyond the bare statement, "This theory T explains physical objects". Likewise, saying "This theory T explains physical objects rather than social symbols" is different altogether from "This theory T explains physical objects rather than physical processes." Another example: "Why do you rob banks rather than robbing houses?" is a radically different question from "Why do you rob banks rather than not robbing anything at all?" It's an interesting question, and for which I don't have any sure answers, as to whether the contrastive character makes for important logical differences when compared with other kinds of reduplicative expressions; but it does seem that every contrastive expression "as opposed to R" is equivalent to the reduplicative expressions "as not-R" and "insofar as it is not-R", although this is not entirely adequate because many contrastive expressions are linked with ordinariy reduplicatives.
Reduplicatives and contrastives can be considered relevance-restrictors. As such they they seem to suggest that the proper logic for dealing with them is a modal logic. In philosophy one most often finds discussion of reduplicatives and contrastives in the context of contrastive explanation (answers to questions of the form "Why X rather than Y?"), but remarkably I can find nothing in the literature linking considerations of contrastive explanations with modal considerations. (Perhaps I have overlooked it?) Possibly this is because philosophers still have the nasty and out-of-date habit of thinking of modal logic in terms of possible worlds, and possible worlds are clearly not relevant here. What is relevant are aspects. In this sense, at least some reduplication seems to work somewhat like a ◊ -- "S qua R is P" is like saying "S in some aspect, namely, R, is P," which is different from saying "S in every aspect is P" and thus is consistent with "S in some aspect T is not P". Diamond modalities break contradictions in a similar way. But other kinds of reduplication, e.g., "Being qua being is the subject of metaphysics," are more complicated, and I'm not even sure where to begin with contrastives in particular.