Would assume SE made this for the curse effect on melee swings. Depending on the amount of -hp/-mp/-movement speed, it could theoretically be useful on 'something' but albeit it never will be used. Retribution damage was probably added due to the name "Ultio" meaning the same thing.
Did some pretty informal testing on this outside Bastok, and it *seems* to increase Retribution dmg by 10-12%~.
Should've probably been /thf and done way more WSs (only did about 6-7 WSs with and without the Grip) for truly accurate numbers, but w/e. x.x Just wanted to give a lil preliminary testing numbers.