I recently saw a production of A Chorus Line and that prompted me to reread this. Mordden wrote decade-at-a-time studies of the American musical, going from the 1920's through the 1970's, and then wrote a follow up volume covering the period from the 1980's into the 2000's.
The 1970's gave birth to, among many others, Jesus Christ Superstar, Company, Follies, Applause, The Wiz, Grease, A Little Night Music, Pippin, Pacific Overtures, Chicago, Bubbling Brown Sugar, The Best Little Whorehouse in Texas, On the Twentieth Century, Annie, Sweeney Todd, 42nd Street, and, obviously, A Chorus Line. And, as I said, those were "among others" - lots of others. Mordden discusses them all.
Of all the productions that Mordden discusses, I saw only one, and that was one that just gets some brief, appended comments - the "relatively lavish Off-Broadway revival [of Pacific Overtures] in 1984 [that] got much better reviews than the original." That doesn't mean that I have not seen other shows that are mentioned in the book; but I did not see the specific productions of which Mordden writes.
That is the problem with discussing theatrical productions of any kind rather than paintings, sculpture, film, or books. If one sees the same movie twenty times over, say, forty years, it will be the same movie each time, however much the viewer's reaction might change. If one sees two different productions of a play, they will definitely differ, perhaps a lot. (Even if one sees the same production more than once, there will be differences.)
So did Mordden himself see every show he mentions? I am sure he did not, so how did he form opinions of the shows he didn't see? I presume the same way everybody else does, but much more intensively. Cast albums, films, television appearances, libretti, reviews, books, newspaper articles, and any other available sources, I would guess.
So why should anyone care about Mordden's opinions?
Because he is smart and knowledgeable and he writes well.
Some of his knowledge is probably necessary for a writer about this field, but far surpasses my knowledge as a viewer of musicals and a reader of Mordden'work. For example, this paragraph about Sondheim's Sweeney Todd:
Sondheim had flirted with this brand of composition before - the rising three-note theme of B-D-E that highlights Pacific Overtures' opening returns to propell its finale. But Sweeney Todd actively reuses or revamps its ur-themes. "The Ballad of Sweeney Todd" has hardly ended when the sailor Anthony enters singing the melody that launched the Ballad, over different harmony. The eerily compulsive ostinato that runs through the Ballad becomes the ostinato for Todd's "There Was a Barber and His Wife," the music suddenly calmed except for the repeated stabbing of an augmented seventh. The Beggar Woman's theme turns into a minuet for the flashback of her rape at Judge Turpin's party. The traditional Dies Irae melody that slams into hearing during the Ballad is inverted for Sweeney's song when he is reunited with his razors. That was a prayer for the dead; this is a salute to the killers.
This would, I think, be meaningless to anyone who is not familiar with the show. Even having seen Sweeney Todd, though, I do not have enough knowledge about music to understand much of this.
But this is far from a typical Mordden passage. Perhaps the thing Mordden does best in most cases is to make shows sound so intriguing that they seem irresistible. This is understandable for hit shows like A Chorus Line or Chicago or other shows as generally respected as Follies or Sweeney Todd. But reading Mordden's material about, say, Mack & Mabel, Cry for Us All, or Lolita, My Love, I truly regret that I will never see them.
Mordden's entire series about the history of the musical is very much worth reading. One More Kiss: The Broadway Musical in the 1970's was intended to be the capstone of the series and is still fascinating.