oblivion yes, morrowind no, and morrowind I think was better without it, to me, you had a lot more choice in what you wanted to say, the problem is, in oblivion and skyrim, you only had a limit amount of choices and replies to stuff, so dialogue was far simpler
because unless they have a heck of a lot of speech per person, which I highly doubt, but then again old republic is fully voiced, not sure how well that was done, as I don't play it. but for me, oblivion and skyrim had poor speech choices, and people ended up sounding similar after a while
so it really depends on how much VO they really add, if they add a lot then fair dos, but VO is expensive too, especailly if the people doing the VO are pros, or else the voices sound silly and poor, on top of that, will the game support other voices other then english? as it means having to add that too, could end up been pretty pricey then.