Does anyone know what the argument against a personal DPS meter is? I can't think of any down-sides to it.

I don't think a party-wide parser would really cause any problems either, honestly. Harassment won't be any higher than it is now - anyone that cares about it already has ACT. At least with a parser in the game the player that gets kicked will know why they got kicked and try to improve so that it doesn't happen again.