Just finished "Catspaw"/"Friday's Child" (S2D1). I'm so used to hearing these cues tracked generically over everything that I hadn't realized how richly thematic they were. All these melodies that I just thought of as nonspecific tunes are actually leitmotifs for specific people or things. The motif that Fried used in all three second-season scores was his Kirk theme, which I never would've guessed since its minor key makes it sound kind of ominous to me, not like hero music. And "Friday's Child" had a Klingon theme and a Teer theme in addition to the obvious Capellan and Eleen themes, etc. Oh, and the baby's theme was Eleen's theme with a faster, less syncopated rhythm. I never realized the distinction before. You'd think I would've noticed the patterns in the episodes -- what was happening onscreen when a particular motif played -- but again, I was already so used to the cues being used generically that it didn't really occur to me. (So that's two recurring themes that I never realized were Kirk themes, one from Steiner and one from Fried.)
There was a lot of unused material in the "Catspaw" score! Mostly a somewhat harsh version of the Courage fanfare that I'm not surprised they omitted, but also a whole leitmotif for the castle with kind of an Olde-English, Robin Hood movie sound to it. That was replaced in the episode with the "Drugged" cue from later, as I recall (or one of the later cues that used the same "brainwashed crew" motif). Maybe it didn't come off as scary enough. So I can see why the substitutions were made, but it's so cool that this unused material is finally getting heard.
Even later Trek tended to alternate between Dennis McCarthy and another composer, rather than having one person give it a go.
Well, more than that. On TNG it was initially McCarthy and Ron Jones, then McCarthy and Jay Chattaway. On DS9 it tended to cycle between McCarthy, Chattaway, and David Bell (they had to add more composers when they had two shows on at once), and eventually they added Paul Baillargeon to the rotation as well for both DS9 and VGR. Those four continued on to ENT, with Velton Ray Bunch being added to the rotation, and Bell eventually dropping out. And in season 4 of ENT, budget cuts meant that McCarthy no longer worked with an orchestra and instead collaborated with Kevin Kiner on synth scores faking an orchestral sound. There were some TNG composers who were used just once, like Fred Steiner, George Romanis, and Don Davis, and some who did 2-3 each, like John Debney on TNG & DS9, Gregory Smith (conductor of the Remastered main title themes) on DS9, and Brian Tyler and John Frizzell on ENT.