Gene used to rewrite scripts himself, you know. I don't know what his bedside manner was in doing this, though.
It's the job of a TV showrunner (which Roddenberry essentially was, though the term wasn't in use then) to rewrite every script so that the series will have consistent characterization, style, continuity, etc. So it's not as if there was anything exceptional about Roddenberry doing it. That was just part of his job.
That's what I've never understood about Ellison's objections to what happened to "City." He should've known going in that television is a collaborative exercise, that he was working in someone else's universe and thus wouldn't get to do things his own way. If he didn't want his words and ideas altered, why agree to participate in a collaboration in the first place?