The theorycrafter in me rather desperately wants one, but I feel that the Terms of Service would need to be adjusted so that harassment includes using parse data to berate and target other players. I've seen people use "illegal" parsers to that effect, and it was one of the most souring experiences I've ever had in this game.

A well-programmed parser would not only count personal DPS contribution, but also the DPS contribution by other players that you may have provided through buffs/debuffs, as well as damage mitigated/healed, TP regenerated, damage taken, and MP used/regenerated. Such a parser would be incredibly complex to code, but would be able to provide players with a much clearer picture of their contribution to a given party than just their DPS/damage done.