Whatever Mr. Gerrold's motivations are for saying the things he's saying, I tend to believe him. His depiction of Gene's fixation on maintaining control of TNG correlates very well with his overall pattern of behavior since at least 1979. According to multiple accounts, during the production of TMP he was a royal pain in the ass as he tried to maintain the same level of control over the movie as he'd had over the TV show in 1966.
After Paramount stripped him of his creative control of Star Trek when TMP failed to be a Star Wars-level hit, Gene spent the next several years complaining to convention audiences about almost everything that the Harve Bennett/Nick Meyer/Leonard Nimoy team was doing with the films. He generally framed his criticism as disappointment that the films weren't living up to his "vision" of Starfleet as a peaceful, non-military organization.
One thing Gene often complained about was the scene in Star Trek II where Kirk kills the Ceti Eel that crawled out of Chekov's ear. He said that the scene was too violent and that Kirk would never simply kill a new lifeform, even one that was repulsive and likely dangerous. Fast forward about five years, where characters on the Roddenberry-controlled TNG also face a situation where fellow officers are being controlled by alien brain worms in the episode "Conspiracy". And what happens? In a sequence that's way more violent than anything we saw in The Wrath of Khan, Picard and Riker blow up Cmdr. Remmick's head, then grimace in revulsion before killing the alien worm that was living inside of him.
Now, you might think that maybe Roddenberry was indisposed when that episode was being made and that Leonard Maizlish or Maurice Hurley were calling the shots. But according to a story told by Richard Arnold on the Mission Log podcast, it was Roddenberry himself! It seems that the original version of the scene was less graphically violent, but when the studio objected to it, Gene deliberately made it even more violent just to show them who was in charge.
When you take all of that, and add the changes to the licensed material in the late-80s as Gene took control of it (or delegated it to his sycophant-in-chief Richard Arnold, whatever you believe) and set out to eliminate stuff like Diane Duane's Rihannsu or John Ford's Klingons that he couldn't take credit for, a certain pattern emerges: to wit, as much as people wax poetic about Gene Roddenberry's Vision being the key element of Star Trek, it was never the most important thing in Gene's mind.
To Gene, the most important thing was that he maintain control of Star Trek. It didn't matter if the show was any good or not, it didn't matter if he had to violate some of his high-minded ideals in the process (as he did with "Conspiracy") it didn't even matter if all the fans turned on him (at a convention during this same period he told an audience that was unhappy with some element of TNG: "I don't make Star Trek for you people. I make it for me."). It didn't seem to matter if Star Trek went right over a cliff, as long as he was the one in the driver's seat. Now, might some of this have been attributable to a deteriorating mental state brought on by his abuse of drugs and alcohol and the strokes he suffered? I think that very likely. But I think we need to keep this in mind whenever people start talking about how Star Trek needs to stick to Gene Roddenberry's Vision. Roddenberry's primary "vision" for Star Trek was himself in control of it. Everything else was secondary.
What Star Trek needs is good stories with believable characters. Are the ideals that Roddenberry frequently espoused (or at least pretended to) important? Absolutely. But I personally believe that we need to separate the ideals from the man.