Hi,
Something that really bothered me during some games is that with empires that have slavery enabled it's not possible to change the setting for a single species. So your superior, psionic, intelligent and extra unity producing main species will always also be working in lower slavery (worker) jobs where other specialized species of your empire would be much more efficent that have extra energy/mineral gain.
With the additional option to set any single species free of slavery further optimizations could be made in one's empire without much effort, which is why I suggest adding this option.
Something that really bothered me during some games is that with empires that have slavery enabled it's not possible to change the setting for a single species. So your superior, psionic, intelligent and extra unity producing main species will always also be working in lower slavery (worker) jobs where other specialized species of your empire would be much more efficent that have extra energy/mineral gain.
With the additional option to set any single species free of slavery further optimizations could be made in one's empire without much effort, which is why I suggest adding this option.