Just intonation isn’t for everyone. Its plethora of fractions can be daunting, and a lot of composers dissatisfied with 12-tone equal temperament simply can’t think creatively in a free field of fractions. Many of these composers have divided the octave, equally, into more than twelve pitches per octave. The great attraction of any kind of equal temperament is that it allows free transposition to any step of the scale. During the 19th century, transposition became so central to compositional thinking that many classically-oriented composers can’t imagine doing without it. Just intonation doesn’t make transposition impossible by any means, but a just-intonation scale limited in its number of pitches will make certain transpositions available, and others difficult.
It must be said, though, from my experience, that working with an unequal just-intonation scale is like carving in wood – the material has a grain to it that gives the artist something to work against. Working in equal temperament is like carving in plastic: every scale step is the same, and the scale doesn’t suggest very much about how to compose in it. Transposition in an unequal scale can lead to very interesting results, with old musical content expressed in new interval patterns. Many classically trained musicians, however, aren’t willing or ready to think in such terms.
That said, various equal temperaments were the usual deviation from 12-pitch tuning from the 1920s until at least the 1970s. Division into 24 pitches per octave used to be considered, in the first half of the 20th century, the most convenient alternative; this is known as quartertone tuning. Other composers have divided the whole-tone into 5, 6, 8, or 12 equal parts, for 30-tone, 36-tone, 48-tone, or 72-tone scales, each of which offers certain advantages. Other, seemingly more eccentric equal divisionsare actually quite natural, such as 19, 31, and 53 tones per octave.