I'd say this has the potential to be a very, very, good thing. It doesn't look like they are doing this half assed either, so it probably will live up to that potential in some ways.
For instance, with a system that was extensive enough to allow the player to control the pitch somewhat like the one Saints Row used the player could create a voice that sounds almost exactly like theirs or to that of someone else that they could actually hear instead of having to imagine, which would help with the roleplaying, not to mention the hilarity potential (ALVIN!!!).
A voice provides tone to the conversation, angry, sarcastic, sad, whatever and the NPCs can properly react to that. With a silent protagonist unless the line was blatantly obvious sometimes you could select a line then find the NPC react in a way that is completely contrary to what the player imagines that line to sound like i.e. "are you okay?" intended to be kind with a response from the NPC like "Enough with the sarcasm! Dick!" It's so much easier to be immersed in a world when you don't have to imagine how your character sounds like and can actually hear what they feel, especially for a person like me that has autism and thus has difficulty understanding how other people are feeling by words and facial expression alone. Speaking of which, the third person view in conversations will also help the character visibly emote in their face and body language.
The dialog interface (which contrary to the claims most people are making actually resembles that of Telltale games much more than it does Mass Effect, the Mass Effect dialog wheel is actually less limited as a matter of fact) both is much less distracting and it's brief summary allows the player to get the gist of what is going to be said and as elsewhere mentioned the general intention due to button position, which makes things less predictable and as the OP mentioned is less insulting to the player's intelligence. It also doesn't seem to be limiting what the typical Fallout games have for dialog options in any given conversation either, as there was very rarely more than 4 options to say to most NPCs in any given conversation besides asking them questions or asking them if you could buy from them anyway and if the conversations so far are any indication there's only 4 options in any given EXCHANGE, not in any given conversation as the player apparently selects each line the Player Character says in that conversation. There's still a few things we don't know about the conversation system too, but we do know that they aren't skimping on the Player Character's dialog with some 13,000 lines each gender.
As for modders, this could actually be a potential boon assuming the game engine allows for it. Modders now have a built in system for the voice acting of the player character in place, meaning that if there was a modder dedicated enough they could record and replace ALL the dialog that is natively there with their own voice acting without having to build in a system for that themselves. I have no doubt the first big thing modders will try to do is mute the voice and replace the dialog interface with classic fallout anyway if Bethesda didn't decide to already, thus having voice acting is potentially the best of both worlds for those who want a voiced protagonist and those that don't.
The main problem with Bethesda games has always been the lack of effectively delivered story, and voicing the protagonist can help with that. The reason for this is Bethesda does in fact write very good stories and characters most of the time, but due to their philosophy that the player must be allowed to do whatever they want they by necessity must write these stories so that if the player decides to take part in the story the Player Character must not be able to do much nor the rest of the world reacting much to what the Player Character is doing. Giving the Player Character a voice gives that character agency in the world, they can affect it and show it affecting them much better that way. With a Player Character with a decently described backstory and an demonstrable if flexible personality Bethesda now HAS to try and make the world, it's quests, and it's main character better written as they cannot nor need to rely mostly on the imagination of the player to compensate for the fact that the player character must allow for the player to do most anything they want anymore.
