I think those role skills are meant to differentiate main tank and off tank playstyle. like the off tank can choose low blow instead of rampart, and majn tank would take provoke and aoe provoke together. If that's so we could find two same jobs with different tools, in the case of 2 pld in the same df party they can just switch role skills inside of duty decide from the start who perform mt role. not optimal, but atleast better than current df.