make right-side gearing for melee mains much much easier. i've mostly played reaper with a bit of dragoon and samurai and monk in there too. this is made convenient by the fact that they all share a right side. while samurai, monk, and dragoon/reaper right sides are not identical you have maybe 1 piece in common beyond them all sharing both the tome ring+raid ring.
if i want to consider raiding on viper, i'll have to invest in scouting accessories which is also shared by ninja and phys ranged which i have no interest in playing. it also just makes more sense for all of one role to share an accessory type