alanback: And most importantly, the FIBS formula is perfectly compatible with the BK formula. I told Fencer long ago that I was able to give him a formula that implemented the FIBS trick for taking into account the length of the match, while leaving the BK formula unchanged for single games. He seemed to be OK with the idea of changing the formula, but not willing to implement it any time soon.

It is not a backgammon-specific problem. The current rating system could also be exploited by playing multi-games chess matches against weaker players.