For immersion purposes. You are actually Gordon Freeman, thus he doesn't talk, you're kind of, 'his voice'. Wrong board.
Almost all character's you play as don't speak. Look at golden sun on gba-the guy you play as doesn't speak, yet when your the other dude, he suddenly becomes mute-and the guy you once was starts chatting away XD
It doesn't feel right for me ethier way... Unless I can alter the charcter's appearance and voice, voice or no voice it feel weird...
The only difference between voice and no voice is if the story is being told to you or you're unfolding the story.