Mr. Pinsky goes on to quote and explicate a number of poems from all periods that have difficulty as their subject matter. That's all well and good, as far as it goes. He says that there is intrinsic value in difficulty:
Difficulty, after all, is one of life's essential pleasures: music, athletics, dance thrill us partly because they engage great difficulties. Epics and tragedies, no less than action movies and mysteries, portray an individual's struggle with some great difficulty. In his difficult and entertaining work Ulysses, James Joyce recounts the challenges engaged by the persistent, thwarted hero Leopold and the ambitious, narcissistic hero Stephen. Golf and video games, for certain demographic categories, provide inexhaustible, readily available sources of difficulty.
Oddly, the poems Mr. Pinsky explicates (by Michelangelo, Shakespeare, and listen favorite Wallace Stevens, among others) aren't especially difficult to understand, which I think undercuts his thesis as I understand it--that difficult art gives pleasure directly because of that difficulty, not in spite of it.
That's not to say that difficulty is inherently a good thing in art, any more than simplicity is an inherent good thing. There are those who would argue both sides of that, as well as their corollaries, that difficulty and simplicity are inherently bad artistic qualities. This is the basis of the Style Wars.
Since music is inherently abstract and poetry is not, the issue of difficulty in music requires a different approach. One parallel does exist, though, in the virtuoso composition, be it a concerto or unaccompanied work. There, difficulty of execution is definitely one of the subjects of the piece.
Pieces of music that pose difficulty of apprehension are a different kettle of fish, and you will often read commentary along the lines of the point-of-view derided by Randall Jarrell, as quoted by Mr. Pinsky:
When a person says accusingly that he can't understand Eliot, his tone implies that most of his happiest hours are spent at the fireside among worn copies of the Agamemnon, Phèdre, and the Symbolic Books of William Blake.
I think Jarrell's tone is unnecessarily snotty, with its implication that anyone who decries Eliot is an utter philistine, but there is a point there. Many people who decry certain what they call excess difficulty in music draw the line at the most "difficult" music that they dig, and say "This far; no further". (It's different when composers do it, because attacking music that's different from yours is a time-honored defense mechanism.) For some, Beethoven is the limit in difficulties, for some, it's Wagner. For some, it's Dylan.
To my ears, music is "difficult" when the ambiguities in the musical discourse exceed what I have learned to process or are of a different nature altogether from what I am accustomed to. When the ambiguities are overcome, the music is assimilated and is no longer difficult, at least not in the same way.
When a critical mass in difficulty is reached, a significant number of composers begin a simplifying movement, and the process starts anew. We can see that happening in recent times, with the rise of aggressively simple (not simplistic or simple-minded, to be sure) tonal music in response to the ambiguities of various schools of High Modernism and Postmodernism in the 50s through the 70s of the century just past. And we are now seeing rising complexity in music that derives from those schools (postminimalism and metametrical msuic, e.g.). And then it will star all over again.
Daniel Wolf has some related posts here and here, and Kyle Gann here.