Voice acting is dirt cheap. I've had whole games and the music done for under 10k. You get to pick who you want and they go in and bang it all out in one day usually.
I don't think voice acting costs are any concern to them at all when they design the game. If it were they wouldn't use any big name actors to achieve their goal.
Then Bethesda shouldn`t be so tight with their quest lines if what you say is true. They should pop in as much speech as Baldurs gate or Morrowind from a good writer and let it go.
I'm not so fond of the good/bad idea either. I think it a bit strange that the game should decide whats right and wrong and judge me, usually because the "good" parts somehow always comes down to a really obvious referens to christianity, which is really misplaced in this kind of game.
For me it's much more rewarding with a strict action-reaction way. If you steel from a rich person, they send mercenaries after you. If you kill a civilian, you get the guards chasing you, and so on.
I try to understand why some younger people seem to have this reaction to anything that talks about good and bad, like they hate the concept. why is this? So they can do anything and not feel remotely guilty? Anyway...
Having good/bad options just helps us have alternatives which can be considered `good` or `bad`. A mother runs up to you and asks you to rescue her little girl from an attacking man. You can respond `yes` or `No`; you may not see that as `good` or `bad`, that`s up to you, but sure looks bad to say `no`.
But there should be a 3rd or more alternative answers such as, "Why is the man attacking the little girl?"
That way you have the `Good`, the `bad` and the `wise` options. Personally i`d add a couple more which would be standard for every situation.
Baldurs gate 1 and 2 understood this well. There was one scenario I remember where we came across a dwarf that openly admitted to being a child-killer and the child-ghost was haunting him. You had the option of a, killing him instantly, b. leaving him alone and c, ask him why he did what he did; which branched to further options.
I always chose to ask why they did what they did first. With the child killer he tried some roundabout psychological crap with me so I would let him off and get rid of the ghost child, but I wasn`t impressed and killed him anyway. Then I went and found the child`s teddy bear which was all the kid wanted to rest.
Now THAT was good branching text lines with good, bad and alternative options. Also a very memorable story too.