I was thinking a bit more on this, and I think that the "Monkey Island" CD games were a great example of enhancing a game through voice acting. The last game was hilarious; much more so because of the excellent voice acting.
Here's an example of the bad: the original PS1 Resident Evil. The voices weren't even synced! Though the dialogue was equally bad, so it wasn't entirely the actors fault, but man, were they awful! Though when it's that bad, it becomes comedy. Barry hands Jill the rocket launcher: "Thanks, I'll take this then." It's not a greeting card, it's a incendiary weapon!!
An old 3DO game, "Off World Interceptor" realized the live-action and voice overs were terrible, and placed two guys a la "Mystery Science Theater 3000" in the foreground to make fun of it. So I suppose awful, truly terrible, voice acting can be a bonus... if you don't take the game seriously whatsoever.