Going by oblivion, you could actually have far fewer voices
Orcs, Nords, Khajiits, and argonians had the same male voice actor, Orcs and Nords had the same female voice actor, Khajiits and ARgonians had the same female voice actor, so for these races the only ones who couldn't share lines are the Khajiits and argonians, since they require sound editing/particular vocal patterns.
Right, and it was terrible. Hearing Lynda Carter's voice (as nice as it is) coming out of every Orc & Nord female svcked.
Based on the good variety in Fallout 3, I'm expecting a better mix of voicework in Skyrim.
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As for the main topic.... fully voiced player character works fine in a game where you have a defined character - you have just one or two voices needed (depending on if it allows a choice of six), and there's at least some "personality" cues to go on in directing the voice actor (example - Mass Effect. Male & Female Shepard, for all the "good" and "bad" dialogue paths, are quite defined - it's easy for them to get some really good voicework in there.)
But in a "make any character you want" game? With lots of choices of race/etc? And no defined background or personality? Fully voiced wouldn't work. And it wouldn't be practical.
(Games like the older D&D games, would give several choices of male & female voice... but since each one was just a selection of battle phrases - like "Die scum!" "Charge!" "I'm wounded!" - you could do that. And there'd be a cutesy voice, a seductive voice, a rough voice, a cultured voice.... you'd pick the one that fit your character. But again - very limited amount of voice acting for each case.)