Famous actors do in small part add to the prestige and pulling power for games though, maybe not with us who play a lot of games and read about them but for the larger market maybe? If you didn't know much about games beyond liking certain ones and disliking some others then having Liam Neason's name thrown around will at least get your attention over other competitors. Okay so it's not like the names are on the box but they can easily come up in conversation. I can picture it paying off, and since it's common practice then it's probably proven to as well, which while taking away from the quality of the game is unfortunatly probably importan enough in a lot of cases to warrent it too. But that said, voice actors can add an amazing amount to a game when they do their job well so if a famous actor manages that with their role then they quite possibly recoup their cost anyway.
By the way, are there vast armies of talented voice actors out there? I have no idea myself, but it does seem like a lot of familiar voices are often cropping up in the games I play even if the actor isn't a famous celeb like Patrick Stewart or Yvonne Strahovski.