For me, voicing is extremely important. Intonation clearly gives off subtler aspects of meaning, something that might be very difficult to convey in plain text alone (or would make said text look silly / goofy). It's a separate dimension, pretty much. Even though English isn't my native language, I always turn subtitles on, but voice is important regardless.
My favourite example in this case are earliest NWN2 OC companions: Khelgar and Neeshka. An absolutely spot-on voicing that gives their characters so much life and realism!
Of course, a bad / off voice ruins a lot, too. Because of that, Astarion for example reeks of fakery for me. I just can't take him seriously!
Voiced protagonists... well, it's a complex topic. In Dark Souls, playing a mute was tolerable, because there aren't that many dialogs in the game, and it's not dialog-centric. On the other hand, in KOAR a silent MC starts feeling weird after a while. Especially considering a superb voice work done with most of NPCs. In DA2 / DA:I, I really liked the voiced MC; what I didn't like was a very slim selection of voice choices during the character's creation. But that's understandable, that would be a lot of extra work for devs.
