From an article by Stephen Greenblatt on Hamlet in the NYRB, almost certainly extracted from his new Shakespeare biography:

Shakespeare found that he could immeasurably deepen the effect of his plays—that he could provoke in the audience and himself a peculiarly passionate intensity of response—if he took out a key explanatory element, thereby occluding the rationale, motivation, or ethical principle that accounted for the action to be unfolded. The principle was not the making of a riddle to be solved, but the creation of a strategic opacity. This opacity, Shakespeare found, released an enormous energy that had been at least partially blocked or contained by familiar, reassuring explanations.

Could not this idea be transposed into a defense of strategies of opacity in other kinds of poetry than the dramatic—specifically contemporary lyric poetry? Consider Greenblatt's closing paragraph in which he suggests a motive for "strategic opacity" beyond provoking "passionate intensity" (deliberate echo of Yeats here?) in oneself and one's audience:

This conceptual breakthrough in Hamlet was technical—that is, it affected the practical choices Shakespeare made when he put plays together, starting with the enigma of the prince's suicidal melancholy and assumed madness. But it was not only a new aesthetic strategy. The excision of motive must have arisen from something more than technical experimentation; coming in the wake of Hamnet's death, it expressed Shakespeare's deepest perception of existence, his understanding of what could be said and what should remain unspoken, his preference for things untidy, damaged, and unresolved over things neatly arranged, well made, and settled. The opacity was shaped by his experience of the world and of his own inner life: his skepticism, his pain, his sense of broken rituals, his refusal of easy consolations.

Greenblatt ascribes Shakespeare's skepticism to his personal tragedy—though the essay strongly suggests that for Shakespeare the death of his son Hamnet encompassed not solely the painful loss of a child, but also the loss of an entire belief system, that of Catholicism and its rituals for the dead which were now forbidden by the Protestant church. That leaves open the possibility that those who choose strategies of opacity in their poetry are expressing their own "deepest perception[s] of existence" which cause them, in our postmodern era, to reject the easy consolations of surface communication and closure. Those most betrayed by the easy pieties of the dominant ideology—feminists for example—have that much more cause to repress "key explanatory element[s]" in their writing. Of course this does not address the question of why Shakespeare's strategy allowed him not only to be true to his own worldview but also made his work compelling to both contemporary audiences and the generations that came after, while a similar strategy applied to poetry has arguably driven away more readers than it has attracted. At least others make that argument; I'm suspicious of claims for poetry's natural popularity that would require poetry to realize itself by stripping away the complexities of language and expression that make it poetry in the first place. Perhaps in the past hundred years or so we've fallen into a game of conceptual chicken, removing not just one explanatory element but more and more of them, until we're left with the fragments that fascinate a few and frustrate others. And theater of course will always have more constraints on it in terms of the time and expense required for its successful production than poetry does (the problem is magnified a hundredfold in the case of film). In the absence of a mass audience we are free to keep raising the conceptual stakes, which a diminishing circle of writer/readers can appreciate. Except that circle doesn't seem to be diminishing, at least not to me: I am more conscious than of the subtle and continuous growth of poetry's utterly passionate audience. So I choose not to despair. And I choose to keep writing and reading toward the limit of my cognitive and emotional capacities, in hopes of expanding those limits until they incorporate me and the unknown world.