And it's not like McCarthy is the first composer to do this. Listen to James Horner's scores, he does it all the time too. Battle Beyond the Stars and Aliens sound just like Wrath of Khan in places. Titanic and Avatar are eerily similar. Hans Zimmer's music all sounds the same.
I'm not getting why it's such a big deal that McCarthy, who wrote music for Trek for 18 years, quoted himself in some of his work.
Heck, listening to the TOS box set, it's evident how much Fred Steiner reused his own motifs. He recycled his Romulan theme as the "Blackship Theme" for "Mirror, Mirror" (and George Duning then used it for Henoch's theme), he reused his leitmotifs for Mudd's women and Andrea in "By Any Other Name," his "Who Mourns for Adonais" score is largely a rehash of themes from "Charlie X" and "The Corbomite Maneuver," etc.
Steiner's and McCarthy's reuses don't bother me much because they're reusing material within the same series or franchise. I don't recall McCarthy doing much reuse across franchises, such as melodies from V
showing up in TNG, though there were a couple of close approximations. I think the reason it's more annoying when James Horner does it is because he does cross franchises rather indiscriminately, like reusing TWOK music in Cocoon