Threat is actually the most logical value to rate players on. For tanks it's their job to generate momentous amounts of threat. For DPS their threat and their damage are equal value unless they're using Quelling or hit something like Elusive Jump (Im actually curious now. Might Elusive Jump near the end of the next Hunt I kill and see if it impacts credit). For healers their threat is equal to their healing+ any DPS they do unless something like Shroud is used. By gauging threat the game is gauging every job's overall output to the battle. The problem isn't with the scoring...the problem is the nature of the fight. Because of the zergfest it's very difficult for DPS to actually push their normal damage numbers (Especially SMN). This hurts traditional parties.