When did Martin write that? I'm wondering which version he used to make the comparison. In the original "Serenade" there were many solos. According to the Balanchine Catalogue, in 1936 a man was added to the waltz, and in 1940 for Ballet Russe de Monte Carlo, all of the female solos were given to a single soloist, with a "supporting female dancer" and the Scherzo a la Russe was added. There were also multiple versions as the solos were redistributed:
Although the steps have remained basically the same, solo measures have been allocated in various ways, most frequently to three ballerinas and two male dancers (New York City Ballet variations have included, among others, five ballerinas [1950, London], four ballerinas [1953, 1955, 1958], three ballerinas ).http://balanchine.or...rchMethod=exact
The original version with changing soloists throughout might have seem less structured and coherent, while the single ballerina version may have imposed a hierarchy that the current version does not, especially when the current version ties together the three female leads in the last movement.