The video did say that changing a companion's class has the side effect of disabling some dialog. Perhaps even whole sections of their personal story. It definitely sounds like a poor idea.
That being said, it is absolutely optional. Don't like it, don't change it. But maybe in someone's fetid brain, it's ideal for Gale to be a barbarian. It doesn't change your experience, so it doesn't matter that they can make that change to theirs.
