Biggest example of this would be Ashley's cowgirl scene with Maria. There wasn't a word of dialogue spoken by the protag during this scene.
You make a good point. My goal is to make the player character a fill in for you, the player. Giving him a strong personality could contradict with the player's personality, or maybe he could say something the player wouldn't agree with. The protagonists are actually the girls, not the player, so these things are deliberate. However in the newest events I've decided to give him more of a voice. But he'll always be "the silent type" so that the player can fill the PC's shoes as much as possible.