playing gammon without the cube is chronologically similar to playing chess without en passant or castling; both were introduced to liven up the game, whether for gambling purposes or not. So to further alanback's analogy, separate ratings to me would be like castling/en passantless chess vs. chess: sometimes a significant difference in game play, but in the end it's still chess. I'll be happy just to wait for ladders.
(hide) If you want to play a game with an opponent of a similar level, you can define a required BKR range for a new game invitation. Then nobody with a BKR outside this range will be able to see/accept it. (Katechka) (show all tips)