Ironically, Fred Freiberger never got a writing credit. Was he rewriting behind the scenes? We don't seem to know. How about Artie Singer? What the hell was he doing? I suspect they were simply rewriting episodes so they could be shot fast and cheap, with no attention to quality.
It's a fact that Freiberger wrote and rewrote episodes of Space:1999 under the pseudoname of "Charles Woodgrove". So, it would not surprise me at all if he rewrote third season Star Trek episodes. On Space:1999, he wrote the episodes "The Rules of Luton" (about intelligent alien plants), "Space Warp" (Actually a pretty good episode), and "The Beta Cloud" (a bug-eyed monster that is actually an alien robot is sent to Moonbase Alpha to steal it's life support system).
Here's a good link to some quality Freiberger info:
http://www.space1999.net/catacombs/main/crguide/vcpff.html
As for Bob Justman, he understood Star Trek very well and had a great track record with regard to science fiction having worked on The Outer Limits. I think he would have made an exceptional producer on Star Trek during the third season. However, no matter who the producer was they were going to be hamstrung with the budget cuts the series faced that year. Some of the stories, I believe were predicated by the budget. Certainly less location shooting was due to that.
But, I think the writing staff would have been in much better hands.
Here's a peak at FFs thought processes:
Martin Landau on Fred Freiberger: He had much less respect for actors and their input and contribution. I'd say 'Koenig wouldn't do that!' Fred would just shrug and say 'It doesn't matter, it's a good script and the audience won't notice if it's inconsistent with his character.' 'Of course they will!' I'd argue for hours and hours... usually losing the battle.
Space:1999 writer, Johnny Byrne on FF: "But, suddenly, we were not talking about Earth people any more. We were talking about some kind of ghastly alternative Star Trekkers: they were Space Men; there was nothing they couldn't handle; they could deal with anything that was thrown at them by aliens—who are inevitably malevolent. Their attitude was, "Get the bastards before they get us. Kick arse quick or they'll kick us." I told Freddie that he was going to lose the sense of wonder that we had had in the first series, and he told me not to worry because he was going to bring wonder into it, so we got a story called The Bringers Of Wonder, and that was Freddy. He completely changed the scripts that I had written, primarily to make it more like Star Trek."
"Freddy made some real contributions to the second series. There was more pace, there was more sense of immediacy, there was more of a believability about some of the characterisations and so on. Because they were written in the main by good writers,
even though many of the scripts were re-written by Freddy, the stories did come out as acceptable and up to the mark. He was a lovable, warm, generous man, but he should have been kept a million miles away from Space: 1999."