nabla: in chess, what about the player who offers draw in a winning position in order to ensure the match win?

Then he wins the match with a slightly worse adjustment to his/her rating. So what? Ratings are just a measurement of your performance - they aren't the goal.

All the interest of cubed backgammon matches comes from maximizing one's chance to win the whole match, while not caring about what the exact score will be.

Yes, that's a difference between *gammon and most other games. A single chess game will always divide a single point between the two players. And typically, chess players are more interested in winning the overal match than in the difference - that's why chess matches seldomly continue if the winner has been decided. A "12 game" match maybe over after 10 games, ending in a 6.5 - 3.5 score. Which is clearly a better result than 6.5 - 5.5.