Good VO can save a game. An otherwise average story can be immensely improved by a great voice cast. Look at Arkham Asylum. It was a great game, but the story was pretty standard comic book fair. However add in Kevin Conroy and Mark Hammil reprising their B:TAS roles and suddenly it feels way more epic than it actually was.
And heck, think about Uncharted without Nolan North or Gears without John DiMaggio. They'd still be fun, but the good VO makes them so much better. I can't say it's crucial really, but done well it becomes a valuable part of the project.