As one of the small fraternity of musical theater fanatics, I have been lucky to marry into a family full of the same. The in-laws are in town and so I’m seeing a lot of shows this week. We kicked things off the other night with “The Drowsy Chaperone.”

It’s a parody of daffy 1920’s shows, and a common response to it is to rue that musicals today are so rarely about pure, innocent joy in the way that bonbons like “No, No, Nanette” were. However, as much fun as it is, the show also demonstrates how marginal the art of crafting a melody has become in American popular music.

Ironically, it’s the score to “Chaperone” itself that points this up. The show’s one flaw is that its score, while witty and serving its purpose, has not a single truly deft piece of melodic writing. In this, “Chaperone” contrasts with a 1954 parody of the exact same type of ’20’s musical, “The Boyfriend,” whose score was actually catchier than that of one of its objects of satire, Richard Rodgers and Lorenz Hart’s “The Girl Friend.”

This was because in 1954, melody still held pride of place in popular songwriting. Sure, there are, properly speaking, melodies in “Mamma Mia!” But I don’t mean just sequences of notes that are more euphonious than the sound of a silverware drawer dropped down a staircase. I mean a sequence of notes that is so beautiful, so “right” that it sounds like it has always existed on high as some kind of Platonic ideal. Jerome Kern was good at this: think of “The Way You Look Tonight.” It’s, quite simply, perfect.

The scores of today’s Broadway hits have given me infinite pleasures over the years — should I admit I’ve seen “Hairspray” six times and fully intend to rack up at least that many more viewings before it closes? — but serious melody has not been one of them. I am aware of not a single truly “right” melody in “The Jersey Boys,” “Rent,” “Spamalot,” “Tarzan,” “The 25th Annual Putnam County Spelling Bee,” “The Color Purple,” “The Lion King,” “Wicked,” “Avenue Q,” “The Producers,” “Mamma Mia!” or even “Hairspray.” And as for how deeply many cherish the music from “Les Miserables,” “On My Own” wouldn’t last 10 seconds in a smackdown with “All the Things You Are.”

It’s the rock revolution that made that kind of songwriting antique. Where our grandparents’ ears were attuned to melody and harmony, rock has trained our ears more to rhythm, vocal charisma, and texture. There isn’t a whole lot to whistle in Radiohead, and to wait for it is to miss the point of their work.

On post-rock Broadway, then, there is the increasing prevalence of what is often termed “American Idol” singing, in which the singer stretches out notes across rippling valleys of improvisatory decoration, for which the technical term is melisma. I am actually a little uncomfortable with those who sniff that these singers are “not staying on the pitch,” as this kind of singing has its roots not in a 5-year-old television show, but in the black gospel tradition, and I for one am not ready to tell Aretha Franklin to “just sing the note as written.” Ms. Franklin’s singing is world-class art.

However, the song itself that is rendered in the melisma style almost never is. It doesn’t need to be: A singer can light up the audience with melisma on the basis of a song with five chords and a melody my cat could write. Whitney Houston could hold my attention just singing the mess out of a single note. But this means that the musical that trucks heavily in this kind of singing — or, in general, on performance charisma rather than what is written — is usually rather light on substance as a written score.

And that does mean something beyond the mere fact that fashions change. Broadway musical scores today contribute much less than before to the body of truly impressive human accomplishment. Fifty years ago today, I and my in-laws could have seen “Candide,” “The Most Happy Fella,” and “My Fair Lady” all in this same week. Their printed scores alone are treasures of artistic achievement. They inspire awe.

This will not be true of “Rent,” “Wicked,” or “The Color Purple” in 2057. Their music pleases via the transient and amorphous matter of performance style. Gratifying as this can be, a finely written score between two covers, shining through even in lousy performances — think of how “Guys and Dolls” scores even when stumbled through by middle-schoolers — is, for my money, more awesome than a performer’s knack for striking a certain visceral chord in my belly one night.

No civilization has it all at any one time. Today, we have TiVo, but building restorers who sculpt terra cotta are a dying breed. One wishes we could have both — just as it would be nice if for every musical score that enchants via performers singing long notes loudly, there was another one that could make the hair on the back of your neck stand up when you play it on the piano on a Sunday afternoon.

Mr. McWhorter is a senior fellow at the Manhattan Institute. His book, “Winning the Race,” is just out in paperback.