"Unique": absolute or relative?

Apropos of nothing, it's always bugged me that, grammatically speaking, the modifier "unique" is not supposed to be modified adverbially (as in "relatively" or "somewhat" or "nearly" unique). Instead, either "unique" is absolute or, by definition, it is simply not unique. I recall reading something by David Foster Wallace about this years ago.

Isn't it the case, though, that nothing is absolutely unique? Rather, anything is unique relative to some qualifier, property, activity, or question. Otherwise, it would follow that everything is unique—because nothing is itself but itself—or nothing is, with the exception of God, who alone (existing a se and in se and thus non est in genere) is actually unique in an absolute sense.

I understand the desire to want to mitigate popular usage of "unique" as a less powerful adjective than it ought to be; used colloquially, and always modified by synonyms of "partially," it comes to mean "pretty different," or "a stand-out among other, similar things."

But the notion of its being absolute, semantically or conceptually, makes no sense to me. I'm no linguist, however, and in any case DFW is not to be gainsaid. Happy to be corrected on this point by the more grammatically enlightened.