OP has a point, anyone who doesn't see it just hasn't been doing newer 'endgame' content as it releases. ARC would be at the center of this debate.. as when Darkhold released it was by far the optimal setup for clearing, and so parties tended to be stacked with ARCs initially. Same issue now.. as ARC was nerfed and has fallen to the lowest spot on the DPS totem pole it is unbelievably hard to just get a group for Ifrit.. not to mention a clear including ARCs.

Once the balancing is all 'done' I imagine the arguement of 'it's totally on the players' will hold weight, but until then, there definetly are STRONG handicap's for including certain DD or healing classes over others, on different content.