It really is a dual-edged blade. In a game where FREEDOM and CHOICES are the focus, voice-acting isn't really the best way to go, unless you have loads of money. Voice Acting in Fallout 3 pissed me off. A lot. There were too little voice actor for too many people. And when the voice actors did different roles, they didn't even try to sound different, they sounded exactly the same. They should've taken the time they spent dealing with the voice-acting and invested it in level design, especially dungeon design, as they're all generic as hell.
But in a game where storyline itself is the focus, it can benefit from the voice acting.
That's the part when people say: "Storyline is the focus in RPGs!", but I disagree on the matter. Some RPGs give more emphasys to player freedom and interaction than storyline construction itself. Once again, I'll use Fallout 3 as an example. Imagine your Fallout 3 playthrough, from beginning to end, with all the choices you made etc. Now imagine that was THE ONLY WAY to play the game. Imagine there was no choices and the character would always do what he did in your playthrough. Now compare the overall storyline of that one (and only) playthrough to that of some other games that are linear, but have a heavy focus on storytelling.
Say... Final Fantasy VII for example (It's not my favorite FF, but I'm using it as an example because it's one of the most "likeable" RPGs). I've never played a RPG that managed to give me total freedom AND had an impressive storyline, full of plot twists and charismatic characters. When they give you too much freedom, they make the story less deep and more generic, because it's totally conditional to your choices. If FFVII was not linear, if it gave you choices, you could've saved Aeris. You'd never have that emotional moment when she dies. And even if you couldn't save her, you wouldn't have that emotional moment, because you'd think "dang, she died, I screwed up at some point, time to reload and do it right this time". But you can NEVER save her, and the fact that you can't just change the story as you like it makes you really get into the story and not just feel like you're "playing with it".
I'm a fan of RPGs, but I honestly enjoy storytelling more than roleplaying.
Anyway, FFVII could benefit from Voice Acting, because it'd add intensity to the scenes. But it all depends on if the voice acting is really bad or really good. FFX english voice acting is so bad, I'd rather play a text only version of the game.