What about when the MC does a dragon shout? Is it in subtitles? Does it ruin the game for you if it isn't the voice you imagined it to be?

Seriously though, in a "build your own character" RPG like Skyrim, most players don't want a specific voice attached to it, because they are either RPing themselves have a specific RP theme attached to it.
IMHO, if the character is predefined however, then it's better if the dialog is voiced...