I read an interesting paper -- well, I didn't really understand it, so it is a stretch to say "I read it" -- but it was by an Engineer at IBM and he was talking about why VBR is preferred for DVD. The jist of what he was saying was that because VBR was more conservative of bandwidth, that left more bw for the picture and when you encoded using CBR, there was a compromise on the video side which was more noticeable since the players could easily decode the VBR audio.  

I'll try to find it and put a link here because what I just said probably doesn't make sense, but what HE said DID make sense and you might like to see it.