Lt. Boma from The Galileo Seven was a really strong character compared to most single appearance crewmen...
Is it possible that they were looking for a new familiar face in Lt. Boma?
What you need to keep in mind is that in 1960s TV, it was commonplace for stories in an episodic TV series to be built around a featured guest character of the week. At the time, the classiest dramas were anthologies, so even shows with continuing characters often aspired to an anthology style, a format that was all about getting the main characters involved with a different featured guest character or characters and telling a story that was largely about the guests.
TOS's first season had a lot of episodes that revolved around prominent guest characters who only appeared once, just like much of '60s TV did. Often these were outsiders or visitors to the ship, like Harry Mudd and Eve McHuron, Charlie Evans, Roger Korby, Miri, the Romulan Commander, or Lenore Karidian; but sometimes they were crewmembers like Gary Mitchell and Elizabeth Dehner, Dave Bailey, Kevin Riley, and Boma. (Remember that Riley's character in "The Conscience of the King" was written as a separate character, Robert Leighton, and then renamed Riley when Bruce Hyde was cast.)
So Boma's prominent role had nothing to do with him being set up as a recurring character, because that's not how TV writers thought in the '60s and '70s. Back then, it was routine for one-time guest characters to be the centers of attention in TV episodes.