I think he means the acting of the main protagonist. I have issues with that as well, because of every thing you say you have it in your head, how it is supposed to sound, tone-wise. With a voice-actor he might not always use the same tone you would have, some interpretations of text are ambiguous.
This is a gripe I have with the dialogue system. Since we only have small scraps of text that we have to extract the meaning from, we do not know what really will be said. Usually it will work alright, but sometimes it might have consequences. I had this issue a few times with the Mass Effect series. I ran into some guy that was really aggressive and the options were basically the following:
1. You looking at me, pal?
2. Uh, sorry I will walk away.
3. Think this trough!
So I am a rational character and chose the third option, figuring he would say something like: "Think about what could happen if we sort this out the hard way." in a diplomatic tone. Instead he said something along the lines of: "Use your head for thinking, moron!" which resulted in violence of course.
Just having keywords has another disadvantage. With your answer written out and you having to read before you decide, you might gain information that was not clear to you before. With only keywords you do not have additional information before you answer. Also you might not fully listen to your own answer and miss something. Happens rarely, of course, but is a thing. Then again only having keywords, if they are chosen well, can really help bring out the essence of answers, so this is an advantage. However I doubt Bethesda can be 100% right, if even Bioware can not fully handle this feature.
Generally, in a game where we usually have a blank character to start with, i.e. Fallout/Skyrim etc. I rather have no main protagonist voice actor, in a game like the Witcher where we play someone else, a rather colourful person I am all for voice-acting. Now there is nothing I can do about it, I just hope that the voice-actor with both audios (german and english) is good enough and meets my needs in conversations.