Absolute concepts such as unique
The rule against allowing degrees of an absolute concept such as unique is based on logic. As Patricia O'Conner puts it, "Nothing can be more, less, sort of, rather, quite, very, slightly, or particularly unique. The word stands alone, like dead, unanimous, and pregnant" (86). Something can of course be almost unique, but it can't be very unique or more priceless or most perfect.
But is logic a strong enough justification for the rule? What about actual usage? Thomas Jefferson, for example, spoke of a "more perfect union," and expressions such as quite unique can be found in reputable sources. Kenneth Wilson points out that "our natural love for hyperbole and intensifiers often leads us to compare some adjectives that conservatives consider absolutes" (4). Though he draws the line atunique, Wilson sees little wrong with an occasional hyperbole such as "Charlie Brown's head is rounder . . . than anybody else's." Wilson Follett also draws the line at unique but accepts expressions such as more perfect and less perfect because "nothing on earth achieves perfection and . . . the degrees of approximation to it deserve to be named" (46).
Roy Copperud argues that there can be degrees of uniqueness. "There are so few unique things under the sun," he writes, "that generally the word is used with a qualifier. This simply extends its usefulness without diminishing its force as an absolute when used alone" (421). The editors of The American Heritage Dictionary of the English Language have found that “resistance . . . may be waning” to using the word with a qualifier such as quite, pointing out that “the nontraditional modification of unique may be found in the work of many reputable writers and has certainly been put to effective use.”
Like many experts, Barbara Wallraff regrets that common usage is robbing the word unique of its meaning. Too often the word is used in the sense of "unusual." "This is too bad," writes Wallraff, "because extraordinary and exceptional and rareand curious and unwonted and strange and peculiar and abnormal and other words as well, in their various ways, all mean 'unusual,' but unique . . . is very nearly unique" (131). Like Wallraff, O'Conner wants to preserve the word's vitality: "If it'sunique, it's the one and only. It's unparalleled, without equal, incomparable, nonpareil, unrivaled, one of a kind. There's nothing like it—anywhere" (86). Mignon Fogarty (aka Grammar Girl) concurs: “There’s just no reason to assign new meaning to unique” (67).
To sum up: Although some experts allow an occasional hyperbole such as more round or less perfect, most of them object—some quite strongly—to expressions that compromise the logical meaning of unique.
Source: Hacker Handbooks (Boston: Bedford, 2013).