You know, I really think I agree that the Shatner ego issue has been overplayed over the years. My theory is that Shatner had no idea what he was doing: it's well documented he used to take the directors aside and say "Wouldn't it be better if Kirk
came to this realisation?", which was really just him beefing up his role as lead actor, putting Kirk front and centre. Where I say he probably didn't even realise is that actors don't tend to read any lines other than their own. Shatner probably hadn't a clue that when he did this, he was actually taking away Sulu's Big Moment In The Script, or he was taking away Uhura's Big Moment In The Script, or he was taking away Scotty's Big Moment In The Script (etc). So naturally, the other actors would seethe that their one (and possibly their only) substantial line in the script had been stolen by Shatner, while Shatner himself probably assumed they had another
scene somewhere else in the script. In some cases, he'd inadvertently taken away Sulu's only line in the episode or whatever, but he was probably clueless that this is what he'd done.