Mel Blanc did so many voices for Warner Brothers that it would be easier to list the voices he didn't do. There was a movie (I think it was Space Jam) where it took a dozen voice actors to do the voices that Blanc did. The multiple voice actor rule sounds like a good idea for the above reason. I could also see them implementing a limitation on the number of voices a voice actor can do on a single show. As an example, in Dragonball Z there is one voice actor (Christopher Sabat) who does so many voices on the show (per Wikipedia more than 13) that in some scenes with three or four characters he is the one who is doing all of the voices. Like losing a voice actor who has become the only voice of a character, losing one voice actor who does so many voices could also create a significant problem.