We believe the benefits of rated bot games outweigh the drawbacks. We are aware that some players may try to farm rating, but unfortunately some users will try to exploit any system regardless of the features we implement.
The rating system is self-adjusting, similar to Elo, so farming cannot continue indefinitely. It is further limited by treating results against the same opponent as a group rather than as completely independent games.
Rating is not intended to be an asset to accumulate, but a tool for matching players of similar strength, because those games are generally the most enjoyable. We understand, however, that many players still perceive their rating as something they own and want to protect.