It's the same way with Slaughter on JP servers. Occasionally there's the odd sucker playing Monk or Dragoon but the overwhelming majority of DPS are BLMs. The mainstay strategy is to hold their LB until they see Sleep going off, then it's a spamfest with Cometeor. I have yet to see a melee DPS or BRD get Battle High. For some odd reason in the last match I saw a SMN get 10 kills, but I bet he switched away from BLM at the last second to get the nexus bonus.

They should divide BLM magic damage by the number of targets in range but that will never happen.