even a turd could be good if you dress it up nice, the idea of giving rdm a seperate skill for buffing magic dps instead of physcial just sounds terrible because of how burst windows work in this game. you don't want to have 2 abilities lock each other out when they're designed with your party in mind. Look at Contagion and Radiant Shield, contagion see's no real viable use outside of pet swap even though it would be phenomenal to have both it and the shield. people will always pick the better of the 2, and if you allow both of them to go up together, then just make one neutral buff. It's terrible skill design.
Double caster is trash, why would anyone in their right mind consider doing triple caster and on that note, we don't need more magic debuffs, we need more jobs that use magic as a point of their core kit, melee/tank/ranged support magic dealers that would allow for magic buffs to be viable. Contagion has the potential to be the most broken ability next to trick and disembowel but not enough jobs synergize with magic to allow for that.