Full VO would only detract from the style of writing going on, with either a narrator eating up most of the lines, or incredibly stilted VO that start seconds before you get to the line being spoken. And if it, then, influenced the way it is written, you'd lose out on a lot of the character in the world; it would suddenly make more sense to go for full cinematics and motion capture, with a third-person camera instead of isometric view. (Mass Effect/Witcher style, basically). This is my long way of saying that the current method of light/important VO only (with the rest text-based) is a good way to do it.
Now to switch topics entirely, the itemisation/loot in Tyranny is really impressing me--it's super good. Item drops make sense, but there is still a progression to the loot (and thus a value to selling it) that so many games get terribly wrong. And they're all useful to certain builds. It actually kind of reminds me of the Spiderweb Software games, only with with more variety. I guess PoE was pretty similar, but I felt they shoveled a lot more onto players in a way that killed the game's economy; in this vein, the broken item solution in Tyranny is genius.
I feel bad for almost skipping Tyranny. I'm having a blast playing it.