First of all, no one is going to do that. Except maybe Uwe Boll.
Secondly, it's not about who is voice acting the characters. It's about the fact that for a story to generally be good, there needs to be some kind of perspective. Even when you're trying to role-play as a character of your own creation, you put a voice to that character in your head because it makes that character seem more real to you. Like they're a part of the world. Most people don't actually notice that they're doing it, they just read the lines in their own voice as if they're reading a book (or as you're doing right now while reading this).
When you give a character in a game a voice, then you're effectively writing a part for that character to play. Rather than playing a choose-your-own-adventure story like you do with a mute character, you're playing as an actual character that "exists" within the world.
Again, look at Mass Effect. You don't really create Commander Shepard... you mold Commander Shepard into a one of multiple forms of Commander Shepard throughout the game. But it's still Commander Shepard.
I think you're thinking about it too technically. No... adding a voice to a mute character does not objectively make the story (meaning writing in your context) better. HOWEVER, if you're trying to create a story that has a certain flow to it in which characters actually interact with each other (which is obviously what Bethesda is trying to do this time), then giving the main character a voice absolutely makes the story better.
Of course, as I said, to do that, you must also sacrifice role-playing... which is the main complain many people here have, and with good reason. Whether or not Bethesda can pull off a happy medium is the question. I hope they can. At the very least, I hope the actual conversation options are a little more expansive than what we saw in the demo, or they'll have removed quite a lot of player choice.