For fighting Ygg, a spatha with weap atk up (not crit chance) and no crit chance on your duals is fine if you use heatseaker - if instead you have crit chance on both duals and spatha and swap out heatseaker you gain maybe 100RP if everything else is the same, so it's not massive. Vs Plume on the other hand I think you always want crit chance on the duals-I haven't done the numbers fully though.