IMO, giving the PC a voice only works in games where the story is more rigidly defined and your PC is a specific individual, with a predetermined role to play.
This^
It works in RDR because you can't choose who you are regardless of whether you want to or not and you can't alter their apperance. You play as John Marston and Rocksrar were therefore able to pick a voice actor suitable for that role.
I will be a Nord and if I was voiced by Gerard Butler (was great in the film 300) or Johnny Depp I would be ok with that, but then again other people wouldn't want them as their voice. So it is an idea that will NEVER work in this style of game